What happened to Eren that made him unable to turn into a Titan in chapter 53 of the manga?
I'm pretty sure this is due to the fact that turning into a titan requires energy.  Serially turning into  one like that was wearing him down, such that he couldn't turn into the normal full-sized titan of normal titan intelligence.  If he burned enough energy as a titan or through regeneration, he would not be able to turn into a titan at all for a while, as established much later in the manga.