I have numerous items to upgrade in my inventory, but I am unable to do so.
I have the materials (items, glimmer, shards).
Item to upgrade will be 265 or so.
Item I am consuming will be 270.
Item to be consumed will say "Base Power Too Low"
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There is a chance the weapon you are trying to dismantle for infusion has a legendary mod on it, something like kinetic damage or a damage type. All legendary mods add a plus 5 light level to your item. So if you've got a 270 and you have a 275 you want to infuse into it, there is a chance that that 275 is actually a 270 with a legendary mod. You can tell by opening the weapon and looking it the mod slot to see if the mod is legendary or not.