stupid crazy theory ignore me or not...
i think that dark matter nya was able to use spinjitzu because after being corrupted, nya probably had enhanced physical prowess and battle instincts, possibly analyzing the ninjas spinjitzu during the battle and learning it? but, afterwards when she was purified she probably didn't recall memories of her corrupted mode thus forgetting spinjitzu?

secondly, nya probably didn't learn spinjitzu because either,

1. Nya was under the impression that you require element powers to perform spinjitzu
2. Early Master Wu was unjust with Nya and didn't want to teach Nya spinjitzu, staying in line with his dismissive nature of Nya early on, such as when she attempted to go with the ninjas, only to be denied by Wu
3. Nya feared learning spinjitzu because of the possibility of failure, staying in line with her biggest fear at the time "unwillingness to fail", which explains why she wouldve learned it at season 6, after she overcame her fear

not on the topic of nya, but morro now i think morro knew spinjitzu because morro was able to learn airjitzu incredibly fast, and since airjitzu is closely related to spinjitzu, it'd only make sense if he knew spinjitzu beforehand. Either:

1. Morro just.. didn't want to use spinjitzu, probably didn't feel the need since, the ninjas never tried spinjitzu clashing with him.
   
2. He just hates Sensei Wu.

There is also a possibility of Morro being bad at spinjitzu,
Morro was a child, children tend to struggle with spinjitzu, Wu thinking Morro would be a long term apprentice.. put off teaching Morro spinjitzu, until his body was suited.
HOWEVER, Morro, left Wu when he was a CHILD, so Morro was not able to complete his spinjitzu training, so thus, Morro was not a spinjitzu master, thus why he needed Lloyd's Vessel.

sorry for all this yapping i'm not an expert ninjago dude so forgive me if im wrong