Since Marriott implemented the feature, I have had %95+ success rate getting e-receipts. Perhaps something is messed up in your MR profile. I suggest calling MR customer service and ask them to check your profile.

--Jon

P.S. The few times I don't get an e-receipt, I typically make do with the paper receipt. If I really needed an e-receipt, I would just call the hotel and ask for one. They should be able to email or fax you the receipt from your recent stay.