Many people have had this problem for varying reasons.
Since others have given the "it'll work out" scenario...
It will soon be 5 years that DH and I have been trying. Just a few months ago we first found out that DH has a prolactinoma, which he'd had symptoms of for over 3 years, but everyone had ignored him. MDs, NDs, just about any healthcare provider, as soon as he said his symptoms then said that he was concerned b/c we'd been "trying", they asked MY age, and shut down. At one point he had a cyst in his chest that was leaking fluid, and the MDs he saw still wouldn't do a hormone level check....
He had to be diagnosed with diabetes to finally find an endocrinologist (that the primary care dude said he didn't need, augh) who could then be urged to do the needed hormone tests. He did, and the supposed diabetes faded (without drugs, the blood sugar levels are very normal now...the endo hardly asks about it anymore) when the dude found the evidence of the prolactinoma (pituitary tumor that causes him to have the prolactin levels of a nursing woman).
Along with that they did an SA (at our urging, again) and checked testosterone, and found that while the SA was better than expected, the test levels were awful.
So, after 4 months of back and forth with our insurance company, they are paying for and providing (through their specialty pharmacy) HCG for DH to inject, to help his body bring his test levels back to normal. They didn't want to pay, they said he should just take testosterone...but taking that would actually CAUSE him to be totally infertile (though he'd likely feel better, more manly), and we didn't think it right that they wanted him to take something that would cause a medical condition. They finally saw the light and are covering it.
Obviously not everyone is going to ultimately find that a pituitary tumor and drastically low testosterone levels are the culprit. But there are those who have tried for years, and nothing has happened.
And the kicker? Just yesterday my MIL decided to CRY to my husband about how our son "needs a sibling" blah blah blah...and she KNOWS that we've been trying since DS was 9 months old....I kinda hate people today b/c of her, seriously, who does that????
But really, 6 months isn't worth being concerned about.
On the other hand, if your husband is gaining squish without gaining weight, if he's getting gynecomastia (aka needin' a bra), if his face looks "soft", etc etc...might want to check those prolactin levels! And don't take no for an answer like my DH did for 3 years of begging health "care" providers for help....