Not much proof of anything, could be just bad luck... but
I would avoid them.
There are lots of dice sites, i would look for a more reputable one
Not much proof? What about the thread "How 999dice.com is stealing your coins, and exactly why you won't believe me" from way back in August of 2015? It might be too long for most people to read, but here's the section that keepinquiet wants you to read.
Until you click that button, you cannot see the hash. If you NEVER click that button, that seed, and the hash that is generated from it, can be WHATEVER THE HELL 999dice wants and needs the seed to be. Are you on a big winning streak, and feeling lucky, and place a huge bet? Are you on a horrifically bad martingale losing streak, doubling your 5 satoshi bet all the way up to 3.35 bitcoin and praying to god you don't lose AGAIN? **DID YOU CLICK THE BUTTON TO GET THE HASH BEFORE YOU BET?**
No? Oh, ok then - sorry - you lost the roll. I guess it was just really bad luck. Do you want to validate that it was actually a losing roll and make sure we aren't cheating? Ok, here's the server seed we used, right there on the hard to find and not explained how to get there validation page, you can use that with the client seed and the bet number and you can validate and see clearly that it all matches up. See? We aren't cheating.
Wait, hold on a second... how the hell do you know that seed wasn't created AFTER you clicked the bet button and the site decided it was time for you to lose? YOU CAN'T. SUCK IT. It is utterly impossible. Because since there's a new hash with every bet, if you didn't request the hash before that crippling losing bet, piss off loser, you can't prove a thing. Once you click bet, if you didn’t copy/screenshot the hash that proves they did not generate the seed AFTER you clicked bet, it's gone. With no way to EVER. PROVE. ANYTHING.
Essentially, unless you're willing to verify everything each time, they could be cheating you.