Why have Yo-Yo's powers stopped working in Agents of SHIELD season 7? After recovering from what happened with the Shrike in season 6 and being allowed to go out in the field again, Elena "Yo-Yo" Rodriguez (Natalia Cordova-Buckley) discovered that her speed powers aren't working correctly anymore, but has yet to understand why.
In the ABC series, Yo-Yo possesses super speed, but her abilities differ from the MCU's other speedster, Quicksilver (Aaron Taylor-Johnson), in one key way. She can move at super speed, but only for a brief moment. After a single heartbeat, she immediately snaps back to her original position, hence the name codename, "Yo-Yo". In season 7, Yo-Yo found herself unable to use move at superhuman speed, but she retained the ability to see and think at an incredibly fast rate. The problem is that she can't move fast enough to react to what's going on around her.
The first time Yo-Yo failed to use her speed it seemed that the problem was psychological, but the second time, it appeared that the situation may be more complicated than that. It could be that being infected with the Shrike had an unforeseen effect on Yo-Yo's physiology. A fan on Reddit speculated that the Shrike altered Yo-Yo's Inhuman DNA. It's Yo-Yo's Inhuman genes that provided her with her powers in the first place, and if the Shrike tampered with her DNA, it could interfere with her abilities.
This theory makes a bit more sense than the other possible explanations. Some have wondered if Yo-Yo could be pregnant and that a second heartbeat could be causing interference, but it's hard to imagine Simmons (Elizabeth Henstridge) not noticing this when she examined Yo-Yo. It's also unlikely that Yo-Yo's new arms are the problem, since they were designed to be compatible with her abilities. Also, Agents of SHIELD season 7 seems to be doing a storyline with Agent May (Ming-Na Wen) struggling with a psychological issue, and the show may not go in the same direction with two of its main characters.
What would it mean for Yo-Yo and the team if the problem was indeed her Inhuman DNA? If this really was the case, it's not clear how the SHIELD team would go about restoring her abilities, but whatever the solution is, there's a good chance that it will be connected with the Kree in one way or another, since it was their scientists who are responsible for the creation of the Inhumans. It's been a while since the Kree have had a presence on the show, but they could make a return during Agents of SHIELD's time travel story. The Kree have played a central role in the lives of Coulson (Clark Gregg), Quake (Chloe Bennet), and the others, so it would seem fitting for the Kree to come back into play for the show's final season.