when installing the pump to the block, i like to use a priming rod [just a length of 5/16" hex rod or long reach t-handle allen key] to check for binding. try the rod with the pump just snug, then torqued up. you can tell immediately if it binds. if so, you need to find out where and why it's doing so.
this doesn't happen all the time, but i have run into this a time or two. so for as long as it takes to check this, it's well worth doing [in my opinion]