Well there are other explanations for this, but none of them are good for me
As Jim mentioned earlier, all of my sieve screens come with a rubber sealing strip at the top and bottom of the screen. The top strip gets pushed against the back vertical wall and keep the dirty water from going behind the sieve screen, and forces it to go the right way over the face of the screen. I do everything I can to make sure each filter is setup right and the screen fits as well as possible before it gets shipped, but environmental issues can cause the tolerances between the sidewalls of the sieve and the edges of the screen to change. If I fit a sieve screen in the winter time and it is 50* in the warehouse, there is a possibility that during the summer months when the water is warmer the plastic inside the sieve will expand and push tighter against the sieve screen on the edges. This will make the screen more difficult to remove, and in some case it could cause the screen to get a slight bow and the top sealing strip could open up a leak between the back wall and the screen. This is one way that waste could be bypassing the screen, but it would not cause a completely clean screen like you are talking about.
Sticking with the thermal expansion theme....Every Zakki Sieve comes with a warning label on the sieve screen that says the filter needs to be protected from sunlight when not in use. If there is not water flowing through the sieve, and the clear domed lid is secured to the filter, you can create a magnifying effect from the sunlight and turn the inside of the sieve into an oven. This extra heat will cause the plastic to swell considerably and deform the filter. I have fixed at least three filters that I can remember due to this issue. If the sidewalls are bent then obviously the screen edges will not sit flush and waste can escape through that gap as well. If this has happened to anyone, please contact me and we can arrange a filter fix for you. I have never charged for this kind of repair, but I can not cover shipping charges so that does get kind of expensive depending on where you live.
Next possibility of waste escaping is the most important one...
The sealing strip at the bottom of the screen rests nicely against the waste tray at the bottom of the screen. I trim the edges of the rubber to fit as nicely as possible, but rubber can swell and cause fitment issues as well. If the rubber is sitting up on one of the welds on the waste tray, then water/waste can escape under that gap and go into the lower section to be sucked into the pump. Pay close attention to how the rubber is contacting the waste tray and if you need to make a little trim to the rubber then grab a pair of scissors and get it done. This is the most important seal, because this is where all of the waste is congregating and if it can find a way out, then it will.
Finally a deformed sieve screen can cause misalignment at the top and bottom so pay attention to that as well. If the screen is bowed then give it some bending pressure to get it straight so it sits and the sealing strips do their job.
Now onto smaller sieve screens...
My sieve screens are not something that can be found on a shelf somewhere. I custom designed my screens after lots of testing with different V wire sizes, angles, slot openings, etc... So that I could get the most water flow through the smallest screen. This makes my screen more expensive to create than the larger screens that are found in other gravity fed sieves. The way I can get this cost down it to buy in bulk so i order about 40-60 screens at a time to get volume discounts. I ONLY purchase standard 250 microns screens during the late orders. So when people ask me for a custom screen I have to be the bearer of bad news and tell them that a single 100 microns sieve screen for the 16" Zakki Sieve will be $550 + shipping and it will generally take 4-6 weeks for it to be produced. So if they want to incur that cost and timeline then I also have to tell them that extra work will be required on their end to make the screen fit properly. I have to shape each screen with a grinder to fit exactly to the filter it is going with. So there new sieve screen will be untouched when it gets to them, and they will have to do the grinding to make it fit properly. Or they could be extremely luckily and get one that fits right out of the box. It is a gamble. This gamble could be removed if they ordered the 100 microns screen right from the beginning so I could fit both a 250 microns and 100 microns screen into the same filter. Like I said earlier, I do not have these screens in stock so your wait time would be extended doing this.
So hopefully I haven't scared anyone away from the filters, but I just wanted to be open and honest that the Zakki Sieve just like anything else can have issues that need to be addressed to make it work to its maximum potential.