What could cause GMAIL messages to simply not arrive?

My son and I are communicating with someone in Russia (not sure that's relevant, but who knows). We've finally figured out that he, using GMAIL simply does not get some of the messages, while I (using Outlook.com) get all of them. I've logged into his account and looked in SPAM and TRASH.... not there. This has happened on and off for a week or more (maybe longer). Any ideas about what could cause this? He has told me this happens with e-mails from other senders, but I haven't verified that for myself and he's not sure it's happening with other senders.

One thought that just occurred to me. Today's missing message had links in it. Could that be a significant variable?

tia
las