Problem Selecting words in my Mac text?

Normally, when I put my mouse and cursor over a sentence to select it, it turns blue then I can take my finger off the mouse, and pull down copy from the Edit menu. suddenly (and consistently) when I lift my finger up, it “unselects” and it is no longer blue. If I try several times I can finally get it to work. But this was never the case before. What would cause this all of a sudden? And how can I fix it please? Is it a problem with my Macbook Pro? My mouse? My email program? Thanks for any help you can give.