I have a few suggestions...
You could always try a reward system....stickers..toys..ect...
Another great idea is try reading to them while they are on the potty and if they go during that time then praise them..treats...hugs...toys...whatever....if they dont go during reading time...move on to asking them to watch a movie while sitting on potty...I am sure you will catch them going in the potty and show them that going in the potty is great!