In order to keep a paper trail of your issue (should you need to pursue legal action), I recommend contacting the attorney who represented you at closing and asking them to draft a letter to the seller. If you did not have legal representation at closing (you closed at a Title company), then draft a letter yourself and send it to the seller via certified mail. Be sure not to make any threats, just give the seller information regarding what was not done per the terms of your contract. Be specific and you may want to have a plumber draft an estimate covering what was not completed. Most likely, the seller will be upset that the person they paid to correct the issue did not complete the job and it will be finished; however, in order for the seller to go back to his/her plumbing contractor, it will be best if they have documentation from you. Good luck!
The first step is to contact the seller and make them aware of this problem. It's possible they are not even aware of this or you may find it is a oversight that they are very willing to correct.
Did you do a final "walk through" of the home prior to closing? It is commonly expected that identified repairs are inspected for completion during this step and prior to closing.