If you play a high note and watch the MIDI LED of the B2M, is there an obvious delay before you hear the synth sound from Garage Band? If so the problem is either the MIDI interface you are using, of audio latency in Garageband.
There is necessarily some delay when converting the lowest notes to MIDI (maybe 40ms on B0), and that can't be avoided. Even on higher notes there is some delay because it has to measure the note pitch and this isn't instant. But this delay is small, and you can adapt to it easily after some practice.
"Missing notes" is usually because the B2M can't see an obvious transition between two notes, and this is more of a problem with the lowest notes. You may need to accentuate your playing so that notes are stopped for an instant before you play the next note -- i.e., in a staccato fashion.
We have some tips here:
http://www.sonuus.com/products_b2m_faq.html