Automatic won't shift into 3rd without "puttering"

My automatic Buick is having a hard time getting into third gear… it happens sometimes in 2nd or first. I don’t know how to explain it, except that it acts like its trying to shift and jerks a bit. Then if I gun the gas it will eventually click in, or if I let off the gas and slowly accelerate, it clicks in- but the problem is getting worse.

How about giving us some more information like the year, model, and engine size of your Buick.