Roku can't play some MKV files created from BDs

[ssybesma]

Roku Media Player seems to play most MKVs fine. Even some ripped from Blu-ray discs.
However, some give the error:
"The codec used to compress the video is not supported."
I've heard it's because the Roku doesn't support MPEG 2 which I guess is the underlying codec on the BDs that gets wrapped by MakeMKV.
The files in the BDMV\STREAM subdirectory all have .M2TS extensions and are AVCHD Video (file type).
What is the easiest way around this problem?

I would need specific instructions if it's complicated.