Often when trying to combine shards, the shard I want to make is highlighted, I can click the hammer button, and it makes the noise of completion, but nothing happens. I can't figure out the logic of it. Problem comes and goes, often I can forge one shard, but not another in the same menu interaction. It's very annoying.
In general, the shard mechanic is kinda lame. So many straight Archaeotech shards drop that by the time you're at a level where you're socketing, you have enough max level shards of all kinds. With Psalm codes, I don't use many shards at all except at all except for on my belt. I end up with half a Stasis page full of lower level shards I'll never use, and only want to combine to meet conditions for some seasonal achievements.