After reprogramming the ignition switch on my GMC Sierra 1999 it still won't start

It sounds to me like the security system is failing i had it happen to me. Its really common in GM vehicles . I did a lot of research after taking it to a mechanic and they wanted an outrages amount to fix it! I found a module that you can install yourself . I put mine in years ago and to this day i have had no issues. They have great customer service and if for some reason it doesnt work they refund %100 of your money. Your vehicle is on the list year make and model for having this problem.. Ask questions they are so helpful .. Check it out i hope this helps ya
http://vatspasslockpasskeysecurityhelp.com/
Do It Yourself GM Security bypass