Hi. I have a 3.5 year old DD. She is currently in the process of being assessed for being on the ASD spectrum as her language development is about 12-18 months behind her peers, and she has difficulties in socialising with her peers and can get 'stuck' playing in repetitive / odd ways (like lining toys up instead of playing with them). Nothing has been officially diagnosed yet but her nursery have raised significant concerns and describe her as having 'additional needs'.
We've been trying potty training on and off for the past year and are getting nowhere. 
We've been trying to explain that she will get a sticker if she does a wee or poo on the potty, which she is enthusiastic about, but she just doesn't seem to be able to manage it. So for example we can sit her on the potty and she won't do anything, but will still urinate / soil herself a few minutes later. Nursery have advised just keeping her in pull-ups and keep encouraging her to sit on the potty regularly since she's quite happy to do this, and 'persist'.
Is there anything more we can do? I try not to compare her to her peers but most other children her age are potty trained by now, and we've been at this on and off for so long now.
Thankfully she has a mid-September birthday so at least she isn't starting school this year, as she'll be in next years intake, so we have some time to work on this.