I had noticed his role on the show had been reduced as well as what has been noticed as mentioned below and now we have an answer. Hope he gets better.
"The Flash, which just recently kicked off its fifth season, has seen Joe's family life become even more tangled. As well as raising a new child he conceived with his new partner, Cecile Horton, he has joined the team in dealing with the arrival of his granddaughter, Nora, from the future. With things gearing up towards the yearly Arrowverse crossover - which will not only feature the first appearance of The Monitor but also see the heroes battle a black-suited Superman - and the landmark 100th episode looming, it would be a tiring time for most people. It's understandable, then, that fans have noticed more and more that Jesse L. Martin has been playing his role from mostly a seated position.
According to TVLine, however, the reason for this is very much based in the real world. Martin reportedly suffered a back injury at some point while the show was off the air. Nothing specific is offered regarding how the veteran actor earned the injury or what treatment is required. Although it didn't preclude him from taking part in season 5's early episodes, it did, however, restrict his presence on the show and is clearly an ongoing issue. As a result, the actor will be taking a leave of absence for the foreseeable future. It's unclear what the circumstances of the departure will be. Similarly, it's unknown how long the character will be away. The show will, however, address the departure directly at some stage."
