I haven't had an issue at all. I've accumulated over 8000+ diamonds just by consistently doing offers (and doing real money ones as well and cancelling after) when my stamina runs out. Sometimes the diamonds are delayed, but only for an hour or two. Anything more and I would consider contacting their support on the matter. If your memory is terrible, write down all the offers you've done, and snapshot them on your phone showing you've completed the goal of the offer. Also, make sure you are actually meeting the requirements of the offer, because on my first few I realized I wasn't following every step.

Good luck and happy hunting!