In a similar position in the past myself I recommend you contact the office of support enforcement yourself. The OSE officers I dealt with in Seattle were very cooperative.
To more specifically answer your question, however, you need to look at the court document that created the garnishment. Garnishments are usually for a specific amount of money (e.g. $Y) that could include past due support rather than just "$X per month until the child turns 18". Sometimes OCE needs to recalculate $Y to stop the order. Sometimes $Y is correct and $X deducted every month will take longer to repay than the number of months until your child turns 18.
If your situaiton is similar to mine part of the payment could be the state recouping benefits paid to the other parent that are not technically child support. EVen if the other parent was not entitled to receive the support. Very frustrating.
Your visit with the OSE should give you all of the information you need as to how much, how long, why that much.
call your local DCS office. if the problem is obvious, they can fix it. if it isn't do a conference board, or hire an atty and go through superior court.
Note: child support often, but not always, terminates when the child is 18. sometimes it terminates when the kid graduates from high school. also, there may be a provision for post-secondary support.
also, the garnishment might be for back support, rather than current support.
read your order of child support carefully to figure out what is going on.