I see it differently.
Even though the lakorn with Son did bad but it was actually good, so I don't think Exact gave her a bad role at all but sometimes, good lakorns just don't do good in ratings. Sometimes you really can't predict the outcome. Some that were made to be hits flop like Nadech and Yaya's lakorns. It's not easy to be in a lakorn with Son na. Son is one of Exact's top pra'akes, so it says something by having Mo paired up with him.
Seems like a lot of the new Exact actresses were being paired up with Pong for exposure so I think they honestly wanted to promote Mo thus gave her a chance with Pong. Although, the lakorn sucked but usually, storylines like that would attract. Too bad, they missed it this time. Same thing with Vill and Charebelle. Even though both lakorns sucked but I actually appreciate Exact in trying to promot them via Pong who is famous in both Thailand and China. It's not easy pairing up with Pong either.
Again, with KNK, the director put in a lot of effort but he fell short. The cast was already good. Toomtam had just finished two hits with Charebelle and Vill, so KNK could have been a hit especially with that type of storyline too.
But Exact lakorns have sucked for the past decade. They've only gotten better since the transition to One HD. Hopefully, it'll get better and better.
I think Mo has been unlucky not because she gets crappy roles but because they just end up not generating enough buzz or ratings.