Sam's kind of a dick
First in "Star Crossed" Sam goes off-mission to fix Donna's love life in the hopes that he can change his own history and marry her. Even though it's against the rules, Al helps him out. We all know how it turns out, history is changed and Sam ends up marrying Donna.
Then in "M.I.A." Al tries to get Sam to help him the same way, change history so that Beth doesn't get remarried while Al is being held MIA in Viet Nam, but oh no, Sam won't do it. He refuses to help Al and Beth marries some other nozzle.
Then in "The Leap Home" Sam again goes off-mission to try and change his own personal history. When Al reminds him it's against the rules he doesn't care and keeps on trying to change things for his family. When it doesn't work Sam runs off like a crybaby and throws a tantrum in a cornfield. "It's not fair!" he cries.
Then he gets another shot in "The Leap Home Part 2" to save his brother in Viet Nam. Sam saves his brother at the cost of the reporter's life and only finds out later that Al was being held POW right there and Sam could have saved him. I can see why Al didn't mention it, he knew Sam wouldn't help him. Selfish Sam.