Hi
I am not a specialist or anything but I noticed that:
- when there are pick-up issues (a loco that has not been used for a while), the sound may come off for a while (a few seconds most of the time)
- 5.5 meters from the power feed seems a lot to me
- specialist recommend a power feed every 2 m of track (good new C track) or even more often with M track or K track where rail joiners may cause a problem
- with C track, the key issues are: rails may be covered with non-visible "machin" causing poor mass return, the studs are with soft iron so prone to corrosion (like M and K Tracks)

I know some specialists will object / scream but I use gade 2000 or finer (Garde 5000) abrasive paper (used with water in the body work industry in the paint finish job) Of course for trains, used without water. The grain is so small that it polishes the rail and studs and removes the "goey" on rails.

Cheers
Jean