I have my own theory base on playing some games from btg pragmatic nolimit and doa from netent. If u aproach about 500spins without bonus u r loged in lost session and propably u will need another 500spins for bonus with terrible return like 10x. What i do now is if play bonanza, money train or doa and reach over 399spins im closing down and open new session. And for some reason on most occasions the frequency of bonuses goes to 3 or 4 per 100spuns(especially doa or dhv).
Btw. Madame destiny and pigi richess megaways i was lucky to trigger features with 1spin only but its almost like win lottery i think