Without knowing more with regard to what you're crafting, these are my ideas on what might be going wrong.
1) Make sure you crafted the correct item. He has a total of three different projects for you to craft.
2) Make sure you crafted TWO of them.
3) Make sure both of the items are in your backpack when you go talk to him.
If you sold the crafting project before going back to Balthazar, head over to the Bazaar where you will be able to buy them back. Make sure to buy two. After you get credit for the quest, you can safely sell them again.
And finally, if you've done all of the Dragonspyre crafting projects, Balthazar may summon you again with instructions to continue your crafting in Zafaria. You won't be able to actually do that until you unlock Zafaria's crafter through questing, however.
Ensure you've the items you crafted (not bought from bazaar) in your backpack when attempting to turn in the quest.
This also goes for all future crafting quests.
P.S - Dworygn, I believe crafters should have a dialogue reminding prospective crafters to walk with the crafted items in their backpack after crafting as so to help diminish this problem stated above.