Problem with Tetris game I am making

I am making a Tetris like game in order to learn Blitz Basic - this is my first real attempt so please forgive the n00b code.

I have got quite a long way (although my code is very messy). I am having problems with my shapes - they are made up of 4 cubes with 1 as the parent entity. If a non-parent cube hits a wall then it goes behind another one of the cubes. If I put collision detection on the actual cubes to prevent this, it has strange effects on my shape as it moves.

Take a look - you'll quickly see what I mean.

Any suggestions on how to fix this most appreciated. Also suggestions on better ways to do things would be useful.

Thanks, here's my code:

[CODE]
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; Main starts here ;
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Graphics3D 800,600

SetBuffer BackBuffer()

Camera = CreateCamera()

CameraViewport Camera,0,0,GraphicsWidth(),GraphicsHeight()

Light = CreateLight()

Dim Block(4)

Block(1) = InitGame()

SeedRnd = MilliSecs()

ShapeType = Rnd(7)

Block(2) = NewShape(ShapeType,2)
Block(3) = NewShape(ShapeType,3)
Block(4) = NewShape(ShapeType,4)

Timer = CreateTimer(5)

Dim Grid(11,20)

While Not KeyHit(1)

Frames = WaitTimer(Timer)

For k = 1 To Frames

x = 0
y = -2

StopCollision = 0
Collision = 0
LCollision = 0
RCollision = 0


For c = 1 To 4

If EntityCollided(Block(c),1)

Collision = 1

If CollisionNX(Block(c),1) > 0

LCollision = 1

End If

If CollisionNX(Block(c),1) < 0

RCollision = 1

End If

If CollisionNY(Block(c),1) = 1

StopCollision = 1

End If

End If

Next


If KeyHit(203)

If LCollision = 0

x = -2

End If

End If


If KeyHit(205)

If RCollision = 0

x = 2

End If

End If

;If KeyHit(200)

; TurnEntity Block(1),0,0,90

; For a = 1 To 4

; TurnBack = 0

; If EntityCollided(Block(a),1)

; TurnBack = 1

; End If

; If TurnBack = 1

; TurnEntity Block(1),0,0,-90

; End If

; Next

;End If



If StopCollision = 1

For a = 1 To 4

If Not a = 1

AdjX = EntityX(Block(1)) + EntityX(Block(a))
AdjY = EntityY(Block(1)) + EntityY(Block(a))

Else

AdjX = EntityX(Block(1))
AdjY = EntityY(Block(1))

End If

ValX = AdjX/2 + 6
ValY = AdjY/2 + 10

Grid(ValX,ValY) = PlaceBlock()
PositionEntity Grid(ValX,ValY),AdjX,AdjY,30
EntityType Grid(ValX,ValY),1

Next

FreeEntity Block(1)

For r = 1 To 20

Full = 1

For c = 1 To 11

If Grid(c,r) = 0

Full = 0

End If

Next

If Full = 1

For c = 1 To 11

FreeEntity Grid(c,r)
Grid(c,r) = 0

Next

For s = r + 1 To 20

For c = 1 To 11

If Not Grid(c,s) = 0

TranslateEntity Grid(c,s),0,-2,0

Grid(c,s-1) = Grid(c,s)

Grid(c,s) = 0

End If

Next

Next

End If

Next

Block(1) = NewBlock()

ShapeType = Rnd(7)

Block(2) = NewShape(ShapeType,2)
Block(3) = NewShape(ShapeType,3)
Block(4) = NewShape(ShapeType,4)

Else

TranslateEntity Block(1),x,y,0

End If

UpdateWorld

Next

RenderWorld


; o = 10
; q = 10
;
; For r = 1 To 20
;
; For c = 1 To 11
;
; If Grid(c,r) = 0
;
; Text p,q,"0"
;
; Else
;
; Text p,q,"X"
;
; End If
;
; p = p + 10
;
; Next
;
; q = q + 10
;
; p = 10
;
; Next

Flip

Wend

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Function InitGame()

For a = 0 To 20 Step 2

x = -10 + a

BottomWall = CreateCube()
PositionEntity BottomWall,x,-20,30
EntityType BottomWall,1

Next

For a = 0 To 40 Step 2

y = 20 - a

LeftWall = CreateCube()
PositionEntity LeftWall,-12,y,30
EntityType LeftWall,1


RightWall = CreateCube()
PositionEntity RightWall,12,y,30
EntityType RightWall,1

Next


aBlock = NewBlock()

Return aBlock

End Function

Function NewShape(ShapeType,BlockNo)

aBlock = CreateCube(Block(1))
EntityRadius aBlock,0.5
EntityType aBlock,2
EntityColor aBlock,100,100,100

Select ShapeType
Case 1
Select BlockNo
Case 2
PositionEntity aBlock,0,2,0
Case 3
PositionEntity aBlock,0,4,0
Case 4
PositionEntity aBlock,0,6,0
End Select
Case 2
Select BlockNo
Case 2
PositionEntity aBlock,2,0,0
Case 3
PositionEntity aBlock,0,2,0
Case 4
PositionEntity aBlock,0,4,0
End Select
Case 3
Select BlockNo
Case 2
PositionEntity aBlock,-2,0,0
Case 3
PositionEntity aBlock,0,2,0
Case 4
PositionEntity aBlock,0,4,0
End Select
Case 4
Select BlockNo
Case 2
PositionEntity aBlock,0,2,0
Case 3
PositionEntity aBlock,0,4,0
Case 4
PositionEntity aBlock,2,2,0
End Select
Case 5
Select BlockNo
Case 2
PositionEntity aBlock,0,2,0
Case 3
PositionEntity aBlock,2,2,0
Case 4
PositionEntity aBlock,2,0,0
End Select
Case 6
Select BlockNo
Case 2
PositionEntity aBlock,2,0,0
Case 3
PositionEntity aBlock,2,2,0
Case 4
PositionEntity aBlock,4,2,0
End Select
Case 7
Select BlockNo
Case 2
PositionEntity aBlock,0,2,0
Case 3
PositionEntity aBlock,-2,2,0
Case 4
PositionEntity aBlock,2,0,0
End Select
End Select

Collisions 2,1,1,1

Return aBlock

End Function


Function NewBlock()

aBlock = CreateCube()
EntityRadius aBlock,0.5
PositionEntity aBlock,0,22,30
EntityType aBlock,2
EntityColor aBlock,150,100,100

Return aBlock

End Function

Function PlaceBlock()

aBlock = CreateCube()
EntityRadius aBlock,0.5
EntityType aBlock,1
EntityColor aBlock,100,100,100

Return aBlock

End Function
[/CODE]