Bulbasaurs are always a tough catch. Typically, they they take five or six great balls, using straight throws. If you find one below about 150cp, it will be easier. If you can throw curve balls, that might make it go a bit faster.
Also, it sometimes seems starter pokies just don't want to be caught. If you start feeling you have wasted too many balls on one, maybe it is time to cut your losses and move on.