It sounds like you're doing everything you can really. Mine can vary too. Every once in a while I'll change things up outside. I used to collect the eggs every single morning when I let them out and didn't seem to get anymore during the day so I started leaving them in the nest until night. Well one of my Austrolorps ended up going broody and staying on the eggs. She doesn't allow anyone else to sit on the nest so in the morning I take the eggs from her nest and put them on the other side of the nesting box(it's about 24X36 with a divider in the middle). I ended up getting more eggs per day. They never want to have 1 nest each, but if they see someone else found a good spot then they'll lay too.
As for the water bucket I just use a 5 gallon bucket we cut down to about 1/3 and they drink from that. The duck we have likes to swim in it so we put in a tall rock we found outside that keeps him from jumping in.
I hadn't thought about neighbor kids that could be stealing them as well. You could try a padlock or even a game camera. My father-in-law recently bought a used one for about $35 that takes pics with or without the flash. He caught pics of the deer out back with them. He even used it to get a pic of neighbor kids that broke into the house and garage.
Could it possibly be that some of the table scraps are too fatty? I had wondered that myself and so I asked that my sister-in-law stop bringing over old pizza, chicken nuggets, and fries. The girls started laying more eggs. I'm not sure if that was a coincidence or not because just days later I found out that I had Infectious Bronchitis spreading through my flock. I was having all the symptoms like sneezing, bumps on eggs, holes in eggs, eggs without shells, deep ridges on eggs, etc. I had every symptom. I even had one where her comb turned almost white so I brought her inside and nursed her back to health. She didn't lay an egg for a week and a half. All are back to normal now and I get anywhere from 4-8 eggs a day out of 9 hens.
Perhaps try to see who is laying what eggs. There's an instruction here on the forum how to insert dye into the vent and find out who is laying and narrow down your problem hens. Oh and I swear it, but when I talk to my chickens often they tend to lay more eggs. You can add me to the crazy chicken lady thread now.
I call my girls brats from time to time because they can be so fickle.
