Problem with veg trug

Hi. I have just joined the forum and would like to know if anyone can help with a problem we have with our veg trug. My husband made the trug for me to grow veg and he made a really good job of it but as time has gone on over the couple of years the back and sides have bellowed considerably and the compost/soil has sunk about 4". My husband left a drainage slit at the bottom, we lined it with black plastic which had holes in it and we added a good layer of polystyrene to aid drainage. Any ideas of what the problem could be please?