What is the best way to stop opposing RB/WR's when they are running to the flat from the backfield?

FWIW, I run a Heavy Man-to-Man (4-3) defense. I don't blitz a lot so my LB can stay in the middle of the field more. My CB's are always in M2M but I play my safeties and linebackers in different types of coverages (M2M, Zone, Zone In...).

Sorry if this has been discussed before, I couldn't find a post about it. Thanks.

Offenses will typically exploit the flat in order to neutralize a strong attack from the defensive line in the middle of the field or to manipulate a defense's strong pass coverage farther down field. 3-4 is stronger against flat routes than your 4-3 for this reason. M2M is also weaker against it since your fast CBs will be out of position to defend them. The way most people decide to do it is getting elite WLB and SLBers who have the speed to close and make sure you don't give up chunk plays.

The fact you recognize flats are an issue for you is a good sign, and means you're reading the game correctly. Your 4-3 and M2M is making it harder for you to defend those flats, and on top of that if you aren't blitzing the correct players they'll be defending a few yards behind the LoS giving that flat route runner a few free yards which could give up a first down.

I didn't know a 3-4 Defense was stronger against flat routes even though it makes sense now that I think about it. And yeah, I always try to get fast Linebackers.

So the only short-term solution I can try is less LB blitzes. I'll take a look if it's worth it to change to a 3-4 Defense next offseason. Thanks for the answer man!