To play back the sample you need to dial up the speed setting.
If you have the speed set to <0% you also have to use longer and multiple grains.
Let’s say you have speed at 50% your grain number needs to be at least 3. This fills the gaps, otherwise you get a tremolo effect. Set grain duration to <50. I always go for 100 to be safe. This lets the sample determine the pitch and timbre rather than the grain generator. (also called a long grain sample)
If you then dial up the random position dial to the left of the wave display, this will help alleviate phase shifting from overlapping grains.