Episparh
I recently added a scope and magazine on my 2nd Executioner without any problem. There is a known issue where the mats do not update after you applied a mod, hence if you try to apply next mod it might show as you have the mats but you actually do not have them. This is why some CMDRs thinks their mats were "eaten" but the mod was not applied. The solution for now is to apply relogski after each mod applied. This will refresh your mats to your current one after previous mod was applied.
I had 3 G5 Weapons (Karma P-15, TK Aphelion, TK Eclipse) with no Mods on them at all. Tried it on all 3, on the Aphelion it gave me the "Processing" circle but nothing was added.
After I relogged I found myself short of a certain type of Mat for whatever reason
Haven't tried it since, went to add Greater Range Mod on both P-15 and Aphelion, my only two Mods so far.