I had the same experience with my now 4 yr. old. All said & done (pediatrician, urologist) we found out that she was actually 'constipated' (which I wouldn't have thought) & that was causing pressure on her bladder which in turn was causing her to have a 'high pressure' bladder. She CANNOT hold her urine, so it sneaks up on her & she'll wet. We TOO were frustrated beyond belief. To make it worse, my 2 1/2 yr old trained like a champ & doesn't wear pull-ups to bed, but her sister (the 4 yr.old) does!! She is now on meds (something to calm her bladder & mirolax for constapation) but it's only been about 3 weeks, so not like MAJOR improvement but small steps!! At least she is not wetting every day-this week only 3 times! And, she seems to be getting to the bathroom in time. This is NO serious problem, but something that needed to be addressed. Not sure if it's something to bring up to your childs pediatrician!!
Good Luck with it-I know TOO well how stressful & frustrating it can be. The sad thing is, we thought she was just being lazy & we'd sort of 'punish' her when it wasn't her fault at all. That made me feel HORRIBLE. But, I think we are on the right track now!!
Best of luck to you!!!