What year vehicle is it?
Mk7 euro 4 just have a single link to the glow plugs fed from a relay, I believe the euro 5 and newer custom/mk8 have a glow plug module that can see all 4 glow plugs. If you've replaced the glow plugs then have you replaced like for like? Or is the glow plug module faulty?
On the mk7 euro 4 there were 2 types of glow plug 4.4v and 11.1v, I was sent the 4.4v and they blew fairly quickly
I've gone by the diagrams with later type so don't really know how the module setup is made.
56 350 2.2 130ps mk7 with 2 side doors
65 290 2.2 125ps custom with 2 side doors
07 350 170ps
Ids v86 licence + v86.04 update + xprog
hereIds map
here