We all know that Harry couldn't repair his wand with this spell. But why didn't he use it to fix his watch? During the second task of the Triwizard Tournament his watch broke while underwater. He never repairs the watch, he just continues to wear it broken. Why didn't he use Reparo to fix it? Could he not fix it with that spell? Has it something to do with the amount of damage, or the way in which it was broken? The watch was damaged by water, not broken, smashed or shattered. Does anyone else know examples of Reparo not working on items besides (wands or the watch) and the condition the spell does work?
Harry took off his watch, which he was only wearing out of habit, as it didn't work anymore, and stuffed it into his pocket.
Harry Potter and the Goblet of Fire, page 543
This is when he finally throws it away, 75 pages after it was destroyed in the water.
History of Magic had rarely gone so slowly. Harry kept checking Ron's watch, having finally discarded his own, but Ron's was moving so slowly he could have sworn it had stopped working too. Page 569
Also I recall Neville breaking teacups in Trelawney's classroom, and she did not repair it either but swept it up and threw it away. Perhaps this was because she was not the best at wand work magic. What do you think?