When a child is difficult to potty train they may not be ready to potty train yet and get out of diapers unless they are nearing 4 years old then you really need to force the issue on the potty training.
If the child is 3 years old or under and not wanting to use the toilet and still wants to wear diapers instead then you should stop trying to potty train the child for a month or two and then just put the child back in diapers and try again.
After a few months remove the diaper and introduce the child to the potty again and they should be ready to potty train.
If they are 3 years old or nearing 4 you should keep up with the potty training and just use training pants and always direct the child to the toilet when they need to pee or poop.
Always take them to the bathroom at certain times and tell them that they have to use the toilet to pee and poop and eventually they will get it.
You may have to show the toddler how to use the potty by sitting on the toilet yourself or if it's a boy have the father show your toddler how to pee standing up by them demonstrating it.
Toddlers are naturally curious and will usually want to copy what there parents are doing so if you let the toddler see you go to the bathroom they will eventually want to do so themselves.
My son didn't potty train until 3 years and 6 months and I just let him see me and his father use the bathroom and he eventually removed the diapers himself when he was wet one day and needed to use the bathroom.
He took the wet diaper off and then went to the toilet himself and used the toilet for peeing and eventually he started pooping on the toilet as well.
He decided on his own that he was a big boy now and didn't want diapers no more.
No matter how difficult your child is to potty train your child will eventually go to the toilet and not want to wear diapers anymore.