You probably need to set up the offsets in the plugin settings.
On the matter of offsets, I have a few suggestions.
I think it would be better if the offsets were calculated in percentages instead of seconds. This would eliminate any possible discrepancies between short and long tracks. The durations would still be calculated in seconds for the sake of having a consistent length for each preview.
Additionally, when previewing a track I think it would be even better to add an additional offset that would read, for example "middle of track", which would be fixed and you would only have to choose the duration. Having three different sections (beginning, middle, ending) would actually give a much better insight into what a track is all about when previewing it.