I have this problem as well. I was sure I was doing something wrong so was going to try again tonight. It was quite frustrating. (Just to clarify: this is in the sample menu).
I also had a separate, unrelated issue where I loaded a new project, added samples, and while the samples play back when I hit the trig keys and I see the line moving, in the source menu, there was no waveform displayed for seven of the eight trigs.