1. Always check your junk folder. maybe your friends have been marked as junk for sending something questionable. Or maybe you previously blocked them for sending silly emails to you.
2. Are they sending mail to the correct address?
3. Is mail forwarding setup for this email? If it is then there will be emails going somewhere else besides your inbox.
4. Is your inbox full?
5. Are the people sending files that are too large for the free email account?
Mitz from Tips4pc