Why doesn't Reparo work with certain items?

Question

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?

Answer 1

This is only speculation, but I think to successfully use Reparo on an item,you need to have at least a basic understanding of how it works. For example, Hermione fixing Harry's glasses in Philosopher's Stone would be perfectly reasonable in this regard, given that only the frame was damaged, while Harry likely has no idea about the inner workings of a watch or the finer details of wand lore.

EDIT: As a comment pointed out, this seems to contradict Harry's repair of his wand using the Elder Wand, but since the Elder Wand is a very powerful and old wand, this could possibly be handwaved, saying that some of a prior owner's knowledge of wand lore was kept by the wand, allowing Harry to repair his wand with it.

Answer 2

We don’t know that Reparo wouldn’t have worked on the watch.

There are certain objects that Reparo doesn’t work on, but that Harry doesn’t use Reparo to fix his watch isn’t necessarily proof that it wouldn’t work on watches. Harry might not have learned how to properly cast it at the time he’d broken his watch. We only see Harry use Reparo in his fifth year.

“He indicated the smashed bowl on the floor. Ron nodded and left.

‘Reparo,’ Harry muttered, pointing his wand at the broken pieces of china. They flew back together, good as new, but there was no returning the Murtlap essence to the bowl.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 15 (The Hogwarts High Inquisitor)}}

Someone not using a spell doesn’t mean it might not work. Reparo definitely does work on teacups. Hermione uses it to repair the teacup Ron used in Transfiguration class when it broke. Whatever the reason is why Trelawney doesn’t use it, it’s not that Reparo wouldn’t work to fix broken teacups.

“He and Ron both tapped the teacups they were supposed to be charming with their wands. Harry’s spouted four very short legs that could not reach the desk and wriggled pointlessly in midair. Ron’s grew four very thin spindly legs that hoisted the cup off the desk with great difficulty, trembled for a few seconds, then folded, causing the cup to crack into two.

‘Reparo,’ said Hermione quickly, mending Ron’s cup with a wave of her wand.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 30 (Grawp)}}

It seems likely that Trelawney just isn’t skilled enough to use it (the only magic we ever see her do is levitating a crystal ball and levitation is first-year magic). While that might not be it since we have no real confirmation, the reason definitely isn’t that it doesn’t work on teacups.