I had a similar sounding problem with my BLI H-10 (https://www.modeltrainforum.com/showthread.php?t=177604).
Ultimately it came down to cleaning the tender wheels. I would strongly suggest cleaning all of the wheels of the locomotive and the tender, including the axles on the tender where the pickups rest against them.
I bought the Woodland Scenics Roto Wheel Cleaner and it has done well, although I know some other people just soak a towel in some alcohol and lay it over the track and run the locomotive over it holding it in place so the wheels slip over the alcohol.
I should stress that I had very little run time on my H-10 before it needed cleaning, so some of this may just be from the manufacturing process.
Ultimately it came down to cleaning the tender wheels. I would strongly suggest cleaning all of the wheels of the locomotive and the tender, including the axles on the tender where the pickups rest against them.
I bought the Woodland Scenics Roto Wheel Cleaner and it has done well, although I know some other people just soak a towel in some alcohol and lay it over the track and run the locomotive over it holding it in place so the wheels slip over the alcohol.
I should stress that I had very little run time on my H-10 before it needed cleaning, so some of this may just be from the manufacturing process.