Mixing O and O-27?

The #1 cause of the issue you're describing is a loss of good electrical conductivity between apparently connected track sections.

Assuming you're using 3 rail O track, the 2 outer rails are typically Common (negative if you prefer) and the center rail is Hot.  So unlike 2 rail systems, there's no electrical polarity concern with loops that return back on themselves.

Solutions are to first:

1. Tighten the rail connections around track joiner pins. This can be done with a pair of pliers.
2. Then add more electrical drops along the track.  Typically using a distribution block to split each of the Hot and Common wires coming off the transformer so that wire pairs attach to the track roughly every 6 track sections.

Okay. That is good news. Thank you.