In general, as players get more Coins, they tend to start betting larger amounts and the results are, inevitably, losing more than they did when they were betting less aggressively. Many players spend their Gems on Party Favor Charms, which is an excellent way to keep your winnings up. In the case of those Charms, however, most do not provide Coins fast enough to cover the highest level of slot game play, so players who rely on Party Favor Charms often start losing when their bets increase past the 200k+ per wager range.
With very few exceptions nobody should be playing slots with 250k wagers per spin on auto-spin. Playing in that manner is almost guaranteed to result is massive losses, sometimes quite rapidly. If that's how you are playing, adjust your wagering by reducing your bet-per-spin OR turn off auto-spin so your overall wagering amount isn't so extreme. Almost every complaint Casino World gets regarding slot payouts, odds, and Coin losses come from players who are wagering in this manner. For most players, it just isn't a good way to play.