When you make your first terrasteel ingot on the Terrestrial Agglomeration plate you cant use spreaders sending manna to it. Other wise the mana-diamond,ingot,peral despawn.

use sparks

What about people who don't have blaze powder??

by the time you make terrasteel you should long have had nether access...

an alternative option is to use potency lens to increase your spreader throughput. but either way this is by design

the terra plate needs nether access itself to create, so I'm confused as to why sparks are a problem :P