Can I have a garnishment filed against me even while making payments?

Back in November, we received a letter stating that we were going to be garnished. They told us the only payment they would accept without garnishment was 500.00 a month. We have paid that every month since without fail (which is difficult but we did it). Today I get a notice from my HR dept that we have a garnishment? Can they do that even though we agreed to their terms?