I've noticed it a few times on mobile, assuming it's a server side issue. In my case I buy, close the app (for offline revenue that doesn't collect with app open [unless you switch to tournament real quick]) but if I do that too fast the server wont register I bought stocks and thus mulls the action.

If I however wait a minute or two after buying, itll go through. You just need to time it properly. I wait anywhere between 1-5 mins accordingly.