MAchida has slowed a bit but the biggest reason he has struggled more because he let non karateka coach him and they mitigated his unique timing and made him some quasi kickboxing fighter.
Ever watch him fight when he is struggling, he goes back for a bit into his karate bouncing style rather than that strange loose kneed John Wayne swaying circling move that he does. He sticks out his lead hand and lets it flop like he is on some promenade.
When he was a younger fighter he would always say that people tried to get him to change his stand-up and he refused. He did not refuse Cordiero and the razor sharp things he did were dulled. In addition, the demand to be more exciting forced him to dilute his style a bit to stay in the pocket.
He made mistakes against Jones by getting in close for too long otherwise he was challenging him at a time Jones was still developing.
Machidas BJJ is club speed. Meaning he rolls like he has all day and he has been taking elbows to end him. His transitions from striking to in-fighting were tricky ending in sweeps and top control. He has swayed away from foot sweeps off his attacks which were a concern for his opponents. His opponents also learned to step off line after their attack to counter his straight left (Shogun, Jones, Weidman).
He is not as old but more, he has given up tools that help him.
