C.H.
I have found with my two boys that you can try as much as you want and as long as you want and isn't going to work until they are ready.
All of a sudden one day (with both of my boys) they just woke up and decided that they were ready to use the potty! It came first with peeing and then a few weeks to a month or so later the other came...lol My first son had and still has a bit of bathroom anxiety and he holds until he binds himself up- especailly when he was first starting out. Anyway, we just continued letting them wear the underwear so it didn't hurt their self esteem since they could pee on the potty and they tried to poop on the potty but they had a lot of poop accidents and everytime they did we both walked into the bathroom and I flushed it into the toilet and reminded them of where it goes and let them flush it down and threw on new big boy undies! It didn't take too long at all til they were ready.
Sticker charts can work great too if you want to encourage them! Maybe after 4 pr 5 stickers they can get a small prize?!
GOOD LUCK!
C.