Board Thread:Questions and Answers/@comment-25322628-20160216013805/@comment-4929778-20160216022510

Maybe the coordinate is like a response thing. Like he has to feel true emotions to use it effective like Elsa in frozen. and since Frieda who had the coordinate for 3 years couldn't use it to that much of a potential, besides memory manipulation, perhaps it might take a little more time for eren to learn. Although maybe Frieda never really trained as she never really had to use her powers until Grisha fought her. Maybe none of the coordinate users really had to assume Titan form except once or twice.

Keep in mind that rod said that eren had to be a Reiss to use it to his full potential, but we don't necessarily know if Rod is a liar or not and we might never know. So maybe since eren now thinks he can never use it or use it but in extremely small amounts he might never get a hold of it. Although Rod did grow up with people who were under the first kings power and maybe the first king was convinced that you had to be a Reiss to have the power.

Although maybe you have to be from the same bloodline to activate it from the beginning and since Grisha and eren weren't Reiss members they couldn't.

(Sorry if I get off track)

Technically he used it twice. Once to kill the smiling Titan and again ten minutes later to sic the Titans on Reiner.

Considering as how eren might have strong emotions soon if you read chapter 78 (I really want chapter 79!!!) then he might get another chance to use it.