" Apply default transition" between two clips not working sometimes

I think I have had this issue since the 15 version or all along  and I do work around it but now I am curious to know why it only does this on certain clips.

Example: I have two clips on a timeline, in Premier Elements 2022 this time, and I highlight the two ajoining clips to apply a default transisiton. The command does not work. However there are little grey arrows in the upper ajoining corners that mean something to this. So if I trim just a little off the end of the first clip these grey markers go away and I am able to apply the transisiton. What are the little grey markers telling me? and how is this affecting the transtion command. Is it something I can correct without trimming the clip?  As usual, thanks in advance for your interest in this post and input you may have.