Legal Question in Criminal Law in Texas

This is a very broad questions and thus difficult to answer.

In giving this answer I will assume that the sheriff is in fact not "upholding the law" by refusing to investigate/pursue an alleged offense.

Law enforcement generally will not (and should not, in my opinion) get involved in cases that are better handled in civil courts (i.e. breach of contract).

If the sheriff is refusing to investigate a legitimate offense, then the best thing that you can do is put your request to pursue the investigation in writing and mail it to the sheriff via certified mail, retaining a copy for your records. (This will probably get some action on the case.)

Good Luck!