Holding on to wee

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Help - this is drivng me nuts!!
I am trying to potty train my daughter (2.4) she wants to wear her knickers and will sit on the potty for long periods of time, but nothing comes out, just before it does she jumps to her feet and starts fussing around to keep it in.
She can hold all morning till her nap then all afternoon till bedtime and a nappy. She is clearly unconfortable by this time crossing legs and dancing about to hold in.
Any tips on how to get her to go.

Bad if they hold on to long, prone to getting Urinary Tract Infections (I speak from experience). We had terrible problems with DD holding on to long, I still worry about the state of her kidneys (could have been long term damage, we don't know until she's an adult).
Chocolate button for select wees? You decide which ones, every 2 hours or so?

up her water intake

I would sit her back down, and tell her to keep trying, she is def aware of the feeling otherwise she wouldn't jump around. I work in a potty training room, and i recently potty trained a little boy, who would react very similar, jump up as soon as he felt it coming, we would sit him back down, and he would get distressed, but when he finally did it, he was so so proud, he did need an adult right next to him to keep him there though.

We give the children stickers for going, and this has always worked well.

Thanks for advice - two (long) days have passed since my post and very little change! After posting Sat night I decided to put DD back into nappies on Sun and forget all about it, but no - now she won't wee in her nappy either! So another day was spent jiggling around and walking with legs crossed until nap time and blissfully unaware weeing in her sleep. Then determind to make some progress I kept her up until she exploded at about 9.30pm, I managed to lift her to get some in the pot - then everyone (still awake)much cheering etc treat (DH gave up a Malteezer a 1st) and bed hoping it would all be remembered in the morning. Today same story. The holding on was what was worrying me in the first place so now reading bidibidi ever 2 hours advice I am thinking ouch!
Water has been going in BoysAreLikeDogs, summer here 37-38 degs all week so just don't know how it is all staying in, Don't do juice, but am thinking desperate times and all that. (prob increase chance of infection though?)
It is painful to watch and has had the opposite effect on me, I keep running to the bathroom every 15 mins.

have you tried running a tap while she's on the potty to encourage it? i just read this in a book last night, it said if their bladder gets overful it can be difficult to then pee...might be worth a try. good luck