Your chops might be off, your tempo might be off, your snare might be off or the guy drumming might be a split second off
1) Check that you have the right tempo.
2) Check that the loop starts on a kick drum which means the snare usually hits on two and four, as in counting one-and-two-and-three-and-four on a 1 bar loop and the kicks on one and three. ( it doesn't have to be like that but that's the usual case and do that if you need to check if the chop is right)
3) If those are as they should and the snare still isn't hitting exactly where they should the drummer might have been a bit sloppy.
You'll learn how to chop right by practicing, you don't have to chop a loop so it starts with a kick but that's the easiest way.
Hope it helped.