Rusev and Lana

This one is not surprising in the least. Since Lana might have shot herself in the foot by announcing her engagement with Rusev while they were embroiled in a feud on opposite sides, their booking hasn't been the greatest.
They did start off as a red-hot act of Anti-American heels. It led to a US Title win and lengthy reign, but as most pushes have, his ended at the hands of John Cena. He did still remain a valuable player in the company after that, but his real chance came last year during the 'Rusev Day' run.
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He was getting the loudest cheers nightly which eventually led to him getting both a WWE Championship shot and a shot at the SmackDown Tag Team Titles. While he failed in both endeavors, the act was still over with the crowd. But unfortunately for him, he was turned heel before WrestleMania. It killed the organic momentum that he had from 'Rusev Day' and he hasn't been used since 'Mania.
Rusev has voiced his frustration before as has his wife, Lana. While both have gripes, Lana is better used as a valet instead of an in-ring talent. Both Lana and Rusev could be booked better and if they were, they'd likely be happier.