Voltage Regulator giving wrong output
The voltage regulator may not be fixed, you should check the actual number on the part of your board, otherwise it may be components placed so they could use either without having to worry about updating there tooling. 

as for what is making it not work, I would check that adjust pin voltage against what is expected in the datasheet, normally its something like 1.2V, If this is less than 1.2V, is the part running hot? its possible there is an excessive load on the output that is causing it to go into a thermal protection mode

Other things you can do would be lifting the regulator and measuring to ground to try and spot a resistive short, beyond this, you would start looking at what is on that power rail, and if any components will allow you to subdivide it into smaller chunks to work out what part of it has gone wrong.