The Bills are getting back exactly what they've put into the OL, which is next to nothing. You can't sign other teams rejects and retreads and expect to be any good. The Bills have repeatedly proven this to be true over the years, consistently fielding some of the worst OLs in the league since the mid 1990s.
To make the point all too clear, all one has to do is take a quick look at the Bills' draft history since 1990. They've drafted only 7 OL on the first day of the draft in the last 16 years. Lets see, that's 7 picks for 5 starting positions in almost two decades. That's a pathetic effort, especially in light of the fact that the Bills OL was falling apart as early as 1994. The only free agents of any consequence that they've bothered with have been Chris Villarial and Joe Panos. Villarial has been a serviceable starter when he's actually healthy. Panos was a flop.
By contrast, the Bills have drafted 13 DBs in the first three rounds in the same time period. That's enough to have turned the entire starting defensive backfield over 3 times, compared to just once on the OL. Of course, I must also mention that 7 of those 13 picks were taken in the first round.
Poor effort equals poor results. The Bills are getting exactly what they've paid for over the years. They've tried to build an OL on the cheap and it hasn't worked and probably never will.