Very low pressure from kitchen sink spray valve why?

Unscrew spray handle and then turn water on if psi is better replace spray handle

Most-likely, you have dirt in aerator ( the attachment on tip of the sprayer ) . Remove aerator, clean it well, reinstall. It should work now...

Unscrew the spray heah and turn on the water if you have a full stream coming out of the hose then check and clear the spray head as Milo suggests. Good luck, Tom