Good call on testing it before screwing it all back together.
Okay, so I saw from your previous post that you'd originally had water damage, so basically it looks like at this point you've replaced almost every major component of your tablet! Wow, you really got into it with this one, didn't you? Happens to me all the time.
Anyway, the obvious places to start are with the LCD connectors themselves. Unplug them, clean them up with at least 90% isopropyl alcohol, and look at them carefully with a bright light and a magnifying glass. Check for any bent or broken pins; if you find any bent ones you may be able to straighten them with fine tip tweezers. Once you're done, plug everything back in and recheck the operation.
BTW, you did unplug the battery first thing, right? Plugging and unplugging LCDs with the power on has been known to fry them.
The only other thing that pops out for me is that you mentioned you've now replaced the motherboard. Was it a known good board? Were you able to verify it worked before putting it in your tablet?
At this point, we don't know if it's an assembly issue or a parts issue. If you got a bad part, it could be the LCD cable, the LCD or the motherboard; it may take some more detective work to figure out exactly what's giving you problems.