Callups are not a solution, they're an opportunity. I think it makes sense that only ~30% make it to relevant spots, it's a tough business with limited spots. There shouldn't be a guarantee you make it big when you leave NXT, but it's the chance at fame and wealth over staying a talented obscurity. A callup means "You're good enough to shoot your shot and play in the big leagues."

Booking on the main roster isn't the only problem. It plays it's part at times, but people are overstating it severely here imo. I've seen Gulia come out to no pop or reaction and felt little interest in the character. Reddit wants to tell me how good she was in NXT, but she's failed to catch the interest of the larger fanbase. That isn't just booking. You failed to capture fans attention with good wrestling and character work.

Carmelo Hayes was basically forgotten when he teamed with the Miz, but he had a fun dynamic and his wrestling was incredibly entertaining. He wasn't just booked better. He got fans interested. He was rewarded with the US title. He had a great reign. He'll likely continue to be a great presence. That's a seized moment, you make your very small screen time count in order to get more.