Bouncing Object

I've cut the code ive got a problem with into this little program:

I Want this image (key) to Bounce as it lands, but when the object hits the ground,
it either dissapears, or moves rigidly up:::why?

Graphics 320,240,16
Global thebuffer=BackBuffer()
SetBuffer thebuffer
AutoMidHandle True

Type dkey 
	Field kx#
	Field ky#
	Field age
End Type
Global drop.dkey
Function DropKey(x#,y#,age=-180)
	drop.dkey=New dkey
	drop\kx#=x#
	drop\ky#=y#
	drop\age=age
End Function


;I've replaced the keys with Red Blocks for this (they have blue shadows)
global keys_shadow=Createimage(4,3)		;Loadimage("key_shadow.png")
	setbuffer imagebuffer(keys_shadow)
	color 0,0,255
	rect 0,0,4,3
global chest_keys=	Createimage(4,8)		;Loadimage("key.png")
	setbuffer imagebuffer(chest_keys)
	color 255,0,0
	rect 0,0,4,3
SetBuffer thebuffer



const RETURN_key=28
While not keyhit(1)
	cls
	
	;press Return to Create a key object
	if keyhit(RETURN_key) DropKey(Rand(20,300),200+Rand(-20,30))
	update_Key_items()
	
	vwait:flip false
	wend
end

Function update_Key_items()
	For drop.dkey=Each dkey
		DrawImage keys_shadow,drop\kx#,drop\ky#
		If drop\age<0
			drop\age=drop\age+1
			DrawImage chest_keys,drop\kx#,drop\ky#+3*drop\age
		ElseIf drop\age<60
			DrawImage chest_keys,drop\kx#,drop\ky#+(Float#((drop\ky#-60)*(drop\ky#))/45)
			drop\age=drop\age+1
		Else
			DrawImage chest_keys,drop\kx#,drop\ky#
			If rectsoverlap(Charx-8,Chary-8,16,16,drop\kx-8,drop\ky-8,16,16) inkeys=inkeys+1 Delete drop.dkey
		EndIf
		
		
	Next
End Function

My Parabola is perfect (as is proven by this little program):

For t=1 To 60
Print ((Float(t)-60)*(Float(t)))/20
Next

Input()
End