PLoni -
It could be that the soil was too lose and the plants have settled. Here is a link with words and graphics that has tons of information.
http://westonnurseries.com/com...
I don't know what other folks may advise, but in my opinion you will have to raise up the plants a bit or you risk losing them to rot. Talk with the people who did the work. Hopefully, they are a reputable company and will want to what's right to correct the problem.
