The chance to land a jackpot (777) is about 1 in 593 spins, and there's also a pity system that starts boosting the odds of landing 7's after your 200th consecutive spin in a single session. I'm sorry the slots have been rough on you, sometimes RNG can be a pain. I'll see what can be done in the next minor update, probably lower the amount of spins required before the pity system kicks in. Thanks for sharing your experience!
Edit: Lowered the amount of spins required for pity system to activate in the recent update.