As a Broadway player, former college professor and current part-time public school teacher of strings, I can think of four possibilities for the problems you speak of. It is possible that you are not holding down the string with your left hand tightly enough to facillatate a firm string divot. Thus, you may think it is the bow, but rather it is not a solid string to fingerboard connection. Next, I would consider that if you haven't re-haired your bow in over two years, you should do so. Bow hair has a specific life, especially if you don't loosen it after you use it. The ablity for the bow hair to hold rosin and do the correct bow thing diminishes over time. I re-hair my bow every nine months to a year. Some of my violin friends laugh at me. The have to re-hair every three to four months. They can hear the difference. It all depends on how much you use your bow and the quality of hair used. I am also wondering if you bought your bow used as it could have had some miles already on the hair. I would consider trying a different rosin. Most pros I know use Carlson or some similar high end brand. They don't switch back and forth between rosins once they find one they like. They get used to the rosin pull and that becomes part of their technique. I personally never liked Pirastro. It never grabbed right with my technique. Lastly, your strings could be shot. Helicore makes a few different kinds of bass strings. If you have on their Pizzicato strings, you will have problems bowing if they are a little old. I have one of my basses set up with their Hybrid sets and I like it very much.
I hope this info helps your bowing issue.
David
