using alpha blending mixer in dsp Builder

Hi

I am working on vip project. I am using dsp builder and Quartus II v6.0.

In the attached figure(dsp builder design model) if i remove alpha blending mixer then design work accuratly.

In attached figure (SOPC Builder) I have just add Nios II core and on chip memory to configure alpha blending mixer.

but I want to introduce picture in picture mixing of 2 layers. background image is 640x480 while foreground image in 64x64 resolution.(here i just want to inert zeros in 64x64)

but in compilation (quartus II)and system generation (sopc builder) is successful.

I cannot understand why there is no output at the vga display.

Can anybody help me, who have use alpha blending mixer?????????

Thank you