Well if it had reached maxium vets for the level, it would not allow you to try to convert more experts to vets.  You will be better off if you wait for a conversion event.  You will have a better % chance to convert during an event.

Vets are capped at 40% of # of sailors. 

The higher the level, the harder it gets to turn an Expert into a vet.

You probably only have bad luck, get a premium account it surely helps a little with the vet conversion rate. :D

Hope this was helpful. 

Pro tip, burn only when it's +300% with premium.

thamks for your answers~It seems that it's better to level up it first:)

by the way is there exsists ability rate limit for a sailor?If it exsists,how many?

Cap ability?
Bunch of em, accuracy, reload, torpedo, restore, repair, sonar and few more.

I have a elite inginer level 78 ( 188 ) veteran = ) maybe i had  good luck

It isn't that hard to get to 180 vets. With the 3x conversion event +50% from premium, you can burn 600- 900 sailors to achieve it from  having already 100-130 vets.

when do u cap with sonar richard? EBVE LVL 120 sonarman?