Guild Wars 2

Yesterday I did 4 combines of rare armor, 3 times I received rare armor back, the 4th time I got nothing back.
Some minutes ago I did 4 combines of rare armor 2 times I received rare armor, 2 times I got nothing back.
It was all armor in the low level 50 range, don’t remember the exact names but I think it’s in the server logs.
I thought you get always somthing back from the forge.

My understanding if you were using all rares is that you should receive 1 rare or a small chance for 1 exo but never nothing back