That's how life goes as a 7th round pick vs. a 2nd round pick. You have to wait your turn and earn your opportunities. Not every 7th round pick can pull a Colston - the stars really have to align for that to happen. Most of them languish in obscurity for several years.
As a 2nd round pick, Matthews has been given several opportunities. He played for two different coaching staffs in Philly and Buffalo traded for him. NFL GMs aren't unlike fantasy GMs in that they do seem to value the unknown over the unexciting. Matthews' last two years are clearly part of a downward slope. If he does enough this year to prove he shouldn't be written off and forgotten then he might earn enough of a role to put up WR3 stats in the slot somewhere. I never said he won't ever be a WR3. I just said the odds are against it at this point.
I think a good comp for Matthews would be Torrey Smith (not due to playing style, but career trajectory and draft pedigree). A fellow 2nd round pick thrown right into a starter's role, his first four years were clearly better than Matthews' first four years. His talent was evident, but not dominant. He's not old (29 - still a WR's prime), but he's largely been a forgotten man since his rookie contract expired. He's still hanging on in the NFL. Started for SF for two years and Philly last year. Has a shot to start for Carolina and put up some points in the Ted Ginn role. But odds are against him being fantasy relevant again, too. A hot start to a career doesn't mean as much as you might think.