Start with the Broker of the agent, make sure they are aware that you are not satisfied, the Broker is responsible for the agents they sponsor. Often talking to the Broker and making them aware of a problem will be enough to get it worked out.
If you do not get the result you are looking for then contact TREC (Teaxas Real Estate Commission) and Tar Texas Association of Realtors...if the agent is not a Realtor TAR will have no authority over the agent.
Let us know what your grievance is and maybe we can help more.
Here are the addresses needed:
For TREC (Texas Real Estate Commission): http://www.trec.state.tx.us/complaintsconsumer/Complaint_ins
For TAR (Texas Association of Realtors):
If she/he is with the Houston Association of Realtors:
Hope this helps.