Expand the time ruler by dragging it to the right. Look for holes, where there is no clip. Eliminate those holes, and see if that helps.
Be sure that the portion of the clip being trimmed is contained in the clip shown in the timeline.
I.E.: If only a portion of a clip is already on the timeline, and you attempt to trim it, the entire original clip may appear in the trim window, including video not on the timeline.
If you then trim a section of that clip that is not already on the timeline you will see only black in the newly trimmed portion.