Rewards Network accepts credit card statements as an alternative to receipts. So you should still be able to get those missing dines resolved that way.

Yes, I'e had more problems with Amex than anything else. Several restaurants where i"d used Amex before I got missing dines until RN apparently fixed the problem (but I was dining there often, and the missing dines took a while, so in some cases I had 2 or 3 dines before it got fixed).

But make sure first that the restaurant is supposed to give miles for Amex payments. There are few restaurants I've seen here and there that don't give miles for Amex payments, even though some of them do accept Amex.