Which keying effect are you using? There are several effects included in Pr for keying, most of which are not very good. Can you show how you have the effect set up? Are you using GPU accelerated rendering?
Ultra key is the best choice for chroma keying.
Video with ultra key set on higher layer. In Your Effects Control Tab , the Opacity Settings value should at 100 with the blend mode set to normal.
Ultra Key (UK) set up should be listed beneath the Opacity & Time Remapping settings. Use the color picker in UK to select the green in your source clip.
Then fine tune settings. I like setting UK's output the Alpha when I am adjusting the quality of Key, then back to composite for preview and normal viewing.
You can prep the footage for keying with fast color corrector, boosting the chroma saturation and heightening the contrast(watch out for over bright whites) a little bit.
Still image on lower layer with no keying added and opacity blending on normal.
Sorry for my late reply..
Actually I use the color key effect.
This works quite well.
Meanwhile I figured out the problem with the alpha channel. There were some effects that didn't work well together.
After deleting all unnecessary effects and after setting up everything from the scratch it finally works.
Thanks for your support...