If you place a fishing net and are not there 10 hours later exactly as it expires, you can’t loot it. I’ve run into this myself, others have reported it as well. If you’re not there as the timer runs down, there’s nothing there when you arrive.
I assume this has to be a bug because expecting us to plan 10 hours ahead, setting timers and making sure we can log in right then, all for a dozen fish, would be patently absurd. (Please tell me this is a bug. Please.)
Can confirm, this is still happening. Third day in a row on two different characters I have set a net, come back the next day and there’s nothing to loot, have to set the nets again.
Not everyone can come back 10 hours later, some people can only come in once a day, this is a dumb mechanic if this is working as intended. I really do hope this is a bug.
Yea this is ridiculous and antithetical to their intention with Dragonflight being an expansion that you play when you want.
This is still happening as of 12/11 hopefully we get a blue post soon
Still having issues here too. 3 days in a row of no fish
two days in a row after daily reset I was only able to reset a new net and there was no location to loot caught fish.
I feel like there is a “catch window” where checking the net yields fish. I’ve checked it well after 10 hours and got fish, but also not got fish. So I assume it’s something like between 10-14 hours (for example) you will get the fish.
But the OPs point stands. If you’re a few hours in the evening type player it could easily be 20+ hours after you set the net 
edit: Well today I just checked the nets and one was empty and the other blocked. So no fish for me. I’d guess about 18 hours after I set them yesterday. Will note down the exact (6:20pm server) time today so I can check better tomorrow.
If this doesn’t change I guess I’ll just not bother with them since they’re literally useless.
Still happening to me. Few days now I set the nets in the evening, come back the following evening, and there are no fish for me.
Confirmed for me as well, still happening. Add this to the absurdly low drop rate on upgrading the fishing nets, and I’m finding them less than useful; they’re almost a waste of time in terms of setting and checking.
For me it seems to have been fixed. I did not come across any unlootable fishing nets I knew I had set of late. (EU)
Interesting, seems like it’s been changed (or there was some other mechanic going on behind the scenes we’re not aware of) cos I just checked my nets about 26 hours after settings them and got fish this time. Either way, much better to actually be able to get fish.
I can confirm as of today the other nets at the other holes are working properly it is just the river mouth fishing hole that is having problems, both nets do not appear after the 10 hour cooldown. You do not have to come right back after the cooldown is up by the way. At the other holes the completed nets will stay after the 10 hours are complete until you loot them.
I really wish they’d stop wasting our time and fix this. 
I’m having same issue
set nets twice now and each time I come back it’s reset. so I get no fish and have to reset nets. I do remember getting fish the first time though.
Please fix this
Just checked the river mouth nets and one is showing as “set fishing net” and the other “blocked net spot”. I’m sure I checked and set at least one of the nets yesterday so yeah whether the nets are full or empty seems to be really inconsistent.
edit: I’d also say that generally speaking the way that fishing quality that affects the nets is bad too. I don’t mean the concept, but the reality that you are reliant on what other people do in order to get a good catch. You can prop up the fishing quality by doing the quests and donating more fish then before you can set the net it’s dropped from good to moderate because there are a dozen people fishing the heck out of the pools.
This needs to be less dependent on the actions of other people because there is a point at which you can’t personally affect it enough to be able to run the nets.
edit 2: Just checked the Roaring Dragonsprings net (for some reason I can only ever set one there) and it was full. I’d assume I set both of them pretty close to the same time yesterday. So no idea why it worked and the river mouth one didn’t.
I have given up upon fishing nets - If I find a net with fish, I am happy. Else I am annoyed. An Indicator where nets are set would help a lot.
Today the normal spot was empty - the “blocked” spot had fish waiting. If I set a net, I set both of them. But the normal net disappeared, the blocked is there.
An acknowledgement would be nice, but it is an annoyance and not an outright exploit that have a higher priority of getting fixed.