If you are still getting 6V instead of 15V then yes the regulators could be to blame .
As they appear to be open metal tag instead of the dearer all plastic construction. then by attaching the metal tab-DIRECTLY without an INSULATOR for the screw you are using to attach the reg. to the heatsink then you could be SHORTING out a connection to EARTH..
Positive regulator= leg-1-IN leg-2-earth leg-3-output
Neg regulator= leg-1-earth leg-2-input leg-3-output
The POS. regulator-if standard-should nave the metal tab to earth-so that would be okay to just use a metal nut+bolt to tighten it to the heatsink.
BUT the NEG regulator has the metal tab connected to the-INPUT-so if you screwed that onto the heatsink WITHOUT an INSULATING washer you would be shorting the input to earth -Not very good-which would put a S/C onto the rectifier reducing its output.
Use your MM to check the tab goes to which leg of the regulators and insulate it
You MUST have been given the parts otherwise the kit is critically short of vital parts.
If as seen it is GLUED to the heatsink VIA the flat insulator thats okay.
But using this type of insulator is never as good as a strong metal bolt through it to tighten it.
Check this out and get back