So a couple of things I've noticed in general.
First if one of the items is barely off the plate the client can think it is working (and show the animation) while the server doesn't. If you wait long enough the items will then despawn.
Second if your plate isn't correct (screen shot looks good though. As long as you have livingstone under the plate itself) once again the client will show the animation but the server isn't actually doing anything.
Lastly, if you run out of mana the process will stall and the client will keep showing the animation until the items despawn.
I don't know if any of this applies to your case though.
My setup has 9 pools feeding the plate and it takes about half of a second to make a piece of terrasteel. If it seems to take longer I pick the pieces up and place again because one of the items is probably off the plate by a pixel.
Oh, if you don't mind the look you can put an opencrate and hopper above the plate to drop items onto the plate perfectly every time. I don't do this because I think it is fugly but it would definitely be more convenient.
Huh, what I do for terrasteel is just put the three mana-items next to each other in my inventory, stand on the plate looking down, mouse over each and hit Q, and run off and let my pool handle the rest.
Edited for clarification.
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