This kept happening to me a few weeks ago with my singer and usually i can fix it quite easily but i just couldn't figure out what the heck was wrong. so after a bunch of failed attempts i just took a whole new bobbin and re threaded it the exact same as i thought i did the defect one and put it in and it worked perfectly fine.
so maybe just get a whole new bobbin and start again!
i'm no expert here, but if you get a thicker yarn for the hat then the holes will be smaller. also, you could use a different size crochet hook. but if you do this the hat will clearly be smaller, you could just add a few extra rounds and that may possibly even it out.