Board Thread:Questions and Answers/@comment-36478503-20180805204858/@comment-25322628-20180826093857

No, the reason Reiner could transform was because he hadn't done so as much as Eren was at the time. If a Titan Shifter hasn't transformed after a long time, then even when injured, they'll be able to change into their Titan form while one that has done so for prolonged periods will not be able to until after they've fully recovered from a previous transformation.