Jacqui,
My son did the same thing. He was potty trained for a short time, but continued to wet during the night and then started wetting during the day all the time. We tried EVERYTHING from praise to bribes to punishments. I'm sad to say that it went on for years as we had no idea what to do. Even our pediatrician did not help. He prescribed Oxytocin and there were short term improvements but they never lasted. I felt like a slave to the washing machine. It cause much stress within the home. We even had a VCUG done to make sure his bladder was formed correctly. At age 6 we Finally went to a pediatric urologist. It was only an insurance co-pay, and I wish we would have done it sooner. It was so helpful to hear that so many children have this particular problem that they have several offices dedicated to it.
First, realize that night time and day time wetting are completed unrelated. Many children, particularly boys, wet the bed until they are quite old. My brother did until he was 14. It is something they must outgrow. Many children are such heavy sleepers that they cannot wake up to use the bathroom; they sleep right through it. Continue to use the pullups and get a waterproof pad for the bed. My son is 8 and wets at night still. It has been a great relief to stop worrying about the night time wetting and just buy the pullups. We only have to wash bedding maybe once a week. We also found the Night Pants brand at Kroger/Smiths work very well and are about $7 a package of 15. They come in a package that looks like blue denim.
Next, here is what the pediatric urologist told me: Children are often so BUSY and distracted that they hold their urine when they need to go. They do not want to leave their activity. They hold it and hold it. Finally when their bladder can no longer hold it, they have an accident. If they do this often, the muscles of the bladder become thicker and stronger. This is not good. As the muscles become stronger, they cause accidents because they are too strong. The Oxytocin he was on also is a drug that is made to solve "urgency"; lets say that you ALWAYS feel like you need to go to the bathroom, well then this medication makes it so you do not feel that way. So it was not a good medication for this situation because my son no longer FELT like he needed to use the bathroom but then these ultra strong bladder muscles are pushing the urine out at every turn. He was wetting 6 or more times a day.
Please go see a pediatric urologist. Pediatricians just may not be up to date on this issue. The first thing the urologist told us to do is to make a "voiding schedule". We taped it up in the bathroom. You must make your child use the bathroom every 2 hours. He will then be made to take time out of his busy play schedule to use the potty. You will avoid accidents. The bladder muscles will start to go back to normal. We saw results within a few weeks but you should keep this up for about a year. When we got lazy with the schedule, we would start having accidents again within a few weeks. Children that age need immediate rewards for doing well. The doctor suggested that every day that our child used the bathroom every 2 hours, they got a small prize (candy, etc.) even if they had an accident that day. They stated not to punish (and I am sorry to say that we had punished at times, because it seemed like he was just being lazy).
The schedule helped immensely. We still had occasional accidents until age 7. So strange, but the very day we moved to a new town, they stopped completely. I personally think that stress contributed largely to this problem. We had a very stressful 2 years trying to sell our home, with Dad working in a different town, and I was always trying to clean the house for showings and I also had insomnia that 2 years and was not a happy person. My insomnia and his wetting seriously immediately stopped once we moved. I would suggest evaluating whether there is stress in the home and seeing if there is anything that can be done to change that.
Good luck! It was a long road for us, but it could have been shorter had I known these things earlier. feel free to message me if you need more info.
Edited to Add: the doctor also stated that 80% of the time, once the daytime wetting is solved, the night time wetting wil go away on its own.