Sound muted

I’m very new to making music and to PO. I just recieved my PO33 KO!.
When I try to put a sample on beat 1, then put one on beat 2, sometimes you cannot hear the second beat. I guess that’s because the first sample is to long. Still, even if I shorten that sample, it keeps on “covering” later beats (sometimes even on beat 3 as wel).
How do I make sure this doesn’t happen?
You may point me to a manual where that’s explained, I’m not too lazy :-).