artwork not showing for some flac files

[jeff.perkins]

I am adding Cover Art to some flac files using metaflac.exe. I have used this for 5 test flac files so far.

The flac file updates seems to work fine. New artwork shows up trouble free in JRiver and Groove  Windows PC music players.

With USB Audio Player Pro on my Samsung S10 phone, the artwork shows up fine as well.

However, with USB Audio Player Pro on my Samsung Tab S2, only some of the artwork shows up some of the time.
- 1 file shows the artwork
- 4 files dont show artwork (instead, the default UAPP image shows up)

I tried removing and re-adding the Artwork, and similar results:
- 1 file shows the artwork (altho this time its a different file with a different picture)
- 4 files dont show artwork

The metadata picture block of the flac file looks normal:
METADATA block #3
  type: 6 (PICTURE)
  is last: false
  length: 20278
  type: 3 (Cover (front))
  MIME type: image/jpeg
  description:
  width: 384
  height: 384
  depth: 24
  colors: 0 (unindexed)
  data length: 20236


Any ideas what's going wrong? 
Or how to fix it?
would it help to send you sample files?
Thanks!

[jeff.perkins]

never mind.
I unsinstalled, then re-installed the app, and reloaded the database. Artwork seems to be working now. Maybe something wrong with cache?