Well, it doesn't look like it'll be an easy problem to sort out. I get the impression that the game code is running since it flashed the screen when you coined up, but the graphics aren't drawing. Is there any sound?
Anyway here's a few suggestions on things to try:
1) Remove the ROM board and double-check that there aren't any bent or damaged pins on the four connectors.
2) If you gently flex or twist the board, does the display change substantially? If so, you may have an SMD chip or two that needs reflowing.
3) If you have access to a CRT, try connecting the board through that instead. CRTs are more flexible than upscalers on what they'll display with an out-of-the-ordinary vertical sync signal, so if you do have a Vsync issue with the board then you might be able to see more of the playfield than you are able to with the OSSC.
4) If you have access to an oscilloscope, probe Pin 14 on the JAMMA edge and make sure you're getting Vsync signals.
5) If you have access to a logic probe, probe the Address, Data and Control lines on the graphics ROMs and make sure that they're being accessed by the graphics custom. I made a tutorial video on probing a graphics ROM with a logic probe that you might find useful if you're not familiar with the procedure.
View: https://www.youtube.com/watch?v=2PAtTIAijeA&t=517s
With Tetris, the ROMs at B9, B10, and B11 are background tiles, and the OBJ ROMs are sprites. If the lines for those ROMs don't show activity then you may have a problem with the graphics custom or some of the other hardware in between.
Schematics are available for that board if you want to check continuity on any of the lines:
https://www.jammarcade.net/files/Schematics/Arcade/system16b_schematics.pdf