Two clips refuse to connect together

I think Shotcut snaps your clip to the playhead instead of snapping it to the previous clip. Or maybe some clip on your other tracks interferes with the snapping.

Try moving your playhead to another position. Zooming the Timeline will also help.
If none of that works, right-click on the blank space between the 2 clips and choose Ripple Delete. Note though that if there are other clips to the right side (on the same track) they will also be moved to the left.

Hi @mueshire
As @MusicalBox says, the clip hangs too close to the playhead. When this happens to me, I zoom the timeline to enlarge the empty space, and it works.