I believe I have found a solution (at least in my case). I had a few dozen albums that no songs can be streamed.
On my MC server, I went to COVER ART -> SAVE COVER ART TO EXTERNAL LOCATION SPECIFIED IN OPTIONS
I check the artwork for all the songs/albums that could not be streamed, and noticed they are all > 1 MB size. I went ahead and used MS Paint "edit" function to resize to 75%, and then all the cover art files became 200-500 KB.
I then went back to my MC server, and for all of the albums, I used REMOVE COVER ART, and then COVER ART -> ADD FROM FILE (using the reduced file). Note, in my case, I made a duplicate (actually, I did a convert with DSP turned on, Output Format reduced to below 96 kHz, PEQ, added my favorite TB_ISONE ... etc, so the converted audio had the audio the way I wanted), so deleting the higher rez art was no biggie. In my case, I have two separate folders of music with two separate libraries.
Anyways, when I replaced all of the COVER ART with <500 KB files, 100% of all my music will stream fine with JRemote. I hope this helps everyone. Of course, the best solution is for JRiver to fix this specific bug.