I assume you're setting the tempo of the song grid to the tempos of the drum loop you're bringing in. Cubasis doesn't yet have audio time stretching so the grid tempo has to match the tempo of the loop.
So, assuming that, there is also an issue that the loops have not been cut to exact bar boundaries. This doesn't matter much as long as when you duplicate them they all start on an exact boundary.
To do that, have the snap set to bars, put the cursor within the loop you want duplicated, hit copy, hit paste. The duplicate will then start on the next bar boundary and everything will play in time. If the snap is set to none then the duplicate will start at the end of the copied loop. If it is a little short or long then the timing will never be in sync.
Hope that helps.