This UA IT recent "upgrade" is a PITA. Someone just mentioned the KTN. This will get a mismatch because UA has jammed your middle initial into the end of your first name in the PNR, which is not how it appears in KTN database. The result is that you can't do OLCI, as well as not get Precheck.
If United has done this (ie, you purchased the ticket from them), then change your name with United. It's a relatively easy process, although you will need to provide proof of your full name (despite them likely having your passport/etc on file!)
If you purchased your ticket from a travel agent, then your beef is with them, not United.