3YO is fine sitting on the toilet but nothing comes out

Hi everyone.  My daughter will be 3 in late September and we are finally making some genuine efforts to get her on the potty.  Her older brother was fully potty trained by 2.5, but he is a totally different kid, and I had a timeline of really wanting him out of diapers by the time his little sister came along.  We started offering the potty around 2, buying training undies etc., but it's been slow going with very little interest in her end.  The last few days she's wanted to wear undies only, so I've been rolling with it, but yesterday she had 7 accidents (pee & poop), despite sitting on the potty as many times.  HERE IS THE ISSUE: she doesn't refuse to sit on the potty, she will even sit for awhile, she just can't seem to figure out how to actually get pee out and into the toilet.  She has only successfully gotten pee into the potty once; every other time she sits there and nothing comes out, and then like 20 minutes later she has an accident.  Anyone else seen this with their children where the issue is actually getting stuff out of their bodies into the potty?  I know some kids hold it for long periods, which is a different issue.  She isn't holding it, but she isn't getting out in the right place either... Thoughts??  Thank you!!