that's true - online dice is a big scam, and, yes - they monitor your every bet and bait&switch once you begin to win;
it is a classic scammer's trick - bait you with a little win and then hope the sucker [you, the player] will deposit larger balance, since he thinks now he is on a winning streak or very lucky, and blam, they snap the trap shut and switch the losing game on your account!
bait&switch scamming is as old as world, and all other methods;
if you don't cheat - you will never win!
All bets can be verified fairness through hash seed you are using. Primedice use a probably fair system which is very transparent. Theres no way they can switch something to control the result since the seed will be corrupted and you will notice it once you verify your bet.
Primedice is one the oldest dice site that is still active. Theres no way that they will operate until now without a problem if they are shady casino. Its normal to experience that way since you are consistently betting while theres a house edge. You will surely lose in the long run with the house edge in play.