help with sphere and a rotating a flat

Hello i got a problem with my programm and maybe anyone want to help me =)

I got a flat and a sphere on it. I want to rotate the flat, if the sphere is on the flat, the sphere have to move up or down.
I don't know what i am doing wrong with the calculating the way the sphere have to move to be on the flat.

;Graphics
Graphics3D 800,600,16,2
SetBuffer BackBuffer()

;Collisions
Const COL_BALL=1,COL_LAB=2
Collisions COL_BALL,COL_LAB,2,2	;sphere-to-polygon, sliding collisions

;Frames per Second
Global fps=CreateTimer(30)

;Camera
camera=CreateCamera()
PositionEntity camera,0,50,-40
RotateEntity camera,45,0,0

;Light
light=CreateLight()
PositionEntity light,0,50,50

;Lab
lab=CreateCube()
Const LAB_LENGTH = 20
ScaleMesh lab,20,3,20
PositionEntity lab,0,0,0
EntityType lab,COL_LAB

mx=0 ;Variable from the labarinth
my=0

;Ball
ball=CreateSphere()
ScaleMesh ball,3,3,3
PositionEntity ball,10,10,0
EntityType ball,COL_BALL
ResetEntity ball

UpdateWorld
RenderWorld

;------------------------------;
;			Main
;------------------------------;
While Not KeyHit(1)

;Wait for Frames per Second
WaitTimer(fps)

;Store Mouse Movement
mxs = MouseXSpeed()
mys = 0;MouseYSpeed()

If MouseDown(1) ;linke maustaste

	;calculate alpha
	If mx-mxs>-20 And mx-mxs<20
		alpha=mx-mxs
	Else If mx-mxs<-20
		alpha=-20
	Else If mx-mxs>20
		alpha=20
	End If
	
	;Coordinates of ball
	ballX = EntityX(ball)
	ballY = EntityY(ball)
	
	;If lab rotates move ball
	If mx <> alpha
		If mx<0 And alpha>0 Then
			yVerschiebung = (Sin(mx)*ballX)
			yVerschiebung = yVerschiebung + (Sin(alpha)*ballX)-(ballY)
		Else If alpha > mx And alpha>0 Then
			yVerschiebung = (Sin(alpha)*ballX)-(ballY)
			If yVerschiebung <> 0
				yVerschiebung = yVerschiebung
			End If
		Else If alpha > mx And alpha<0 Then
			yVerschiebung = (Sin(alpha)*ballX)-(ballY)
		End If
	End If
	
	;move ball
	If yVerschiebung > 0 Then
		If EntityX(ball)>0 Then
			MoveEntity ball,0,yVerschiebung,0
		End If
	End If
	yVerschiebung = 0
	
		
	;X-Axis
	If mx-mxs>-20 And mx-mxs<20
		mx=mx-mxs
	Else If mx-mxs<-20
		mx=-20
	Else If mx-mxs>20
		mx=20
	End If
	
	;Y-Axis
	If my-mys>-20 And my-mys<20
		my=my-mys
	Else If my-mys<-20
		my=-20
	Else If my-mys>20
		my=20
	End If

	RotateEntity lab,my,0,mx,True ;True = relative to 0,0,0 origin
EndIf

If MouseDown(2) ;right mouse, reposition ball
	PositionEntity ball,10,10,0
End If

MoveEntity ball,0,-1,0

UpdateWorld ;check collisions etc.
RenderWorld ;Render the scene
Flip ;Flip Backbuffer

Wend 
;------------------------------;
;		End Main
;------------------------------;