Help on tricky buyer's broker situation

Need a bit of advice on a buyer's broker situation. Broker A has been showing me a bunch of apartments (around 10 to date). However, I have been less than impressed with him. He doesn't know answers to questions which he should (I eventually had to figure out the answers myself), and has been slow at returning calls. In the past two weeks I've seen a few other apartments with Broker B, who has so far been much more reliable. If I wanted to now make an offer on one of the apartments I originally saw with Broker A, am I stuck with doing so through him? Nothing in way of any sort of a buyer-broker agreement has been signed. I would rather make the offer through Broker B as I suspect he'll do a better job getting me through the board approval process. Any thoughts?