switching up a beat?

I hope you guys don't mind me asking a non-MPC question (still saving). I've been at making beats for about 6 months now and I'm at a point where I have no problem putting down a 2 or 4 bar sample with a drum pattern. however, i can't put a whole song together because when i want to switch the beat up (for a chorus, for example or even just to keep things fresh), i can't get the two pieces to sychronize. what am i doing wrong? is there a trick to this or do i just need more practice or better equipment? (i'm only working with a boss sp-505). also, i've been trying to take two or more pieces of a sample and splice them together but the drums from the song seem to get in the way more often than not. is there a way around this? I can't seem to find any site like this for sp-505. Just wondering if someone could help me out even if you have to explain it's done in MPC terms.

try copying the sequence to a new sequence and make sure the bpm is the same.