for what it's worth:
a. not all blades made are actually the same length. Even OEM blades can be off as much as 1/16 - 1/3 inch. It happens. ALWAYS measure the blades...all of them. out of a hundred, some will not be correctly sized, even if sold as an equivalent or even OEM from the same mower manufacturer. You tend to see this is "batches"...blades sold as a set. if there is a dimensional error, all of them will usually be too short.
b. multi blade mower must have a slight overlap...this ensures there is complete cut swath from one blade to the other, leaving nothing standing.
c. some mower designs are poor and the overhand is just not adequate. Cub cadet is famous for this /s There is not enough overhang in the blade swath design...this design flaw reveals itself when turning the mower. straight lines...okay..but when turning, the overhand design flaw reveals it's ugly design.
d. too much or too little downward pitch in the leading edge (forward side) of the deck can cause irregular cutting issues. I pitch all my mowers for a clean smooth cut no more than a 1/3 inch dip front to back. Leveling the deck is important also. make sure you are on a flat even surface when doing this measurement. Sometimes the front deck grass wheels get worn on one side or both, or are loose, or missing. You must have these wheels to prevent scalping and irregular cuts.
e. mowing too fast contributes to irregular cutting...slow down and see if this improves the cut
f. blades need to be sharp...not knife sharp, but reasonably sharp. sharpen the blades on a regular basis. sometimes blades are warped or broken...heat and beat to straighten warped blades...replace broken blades. Most of the cut is done out at end of the blade...but the inner side (closest to spindle bolt), should also be sharp, because this helps to discharge the grass, and cut it into small pieces.
g. mulching blades and the discharge plug is notorious for clumping. consider mowing with standard blades with medium lift.
h. grass that is too high, needs to be cut at a higher height. only the top commercial mowers with the best design and power can go through the very thick tall stuff in one pass. If you cut at a higher mowing height and the swath is cleanly cut, that's the problem. And yes, this means either mowing more often before it gets tall, or two passes...first higher, last lower.
i. not all grass is the same. bermuda cuts differently that zoysia and saint augustine cuts differently also. They grow at different rates. Different species/variant also contain less or more moisture. Grass that is very dry, will be hard to cut cleanly. Lightly moist and "green" grass tends to cut the best. (my theory is that the higher moisture content makes the grass more plastics than dry fiber and that the moisture acts in a way that is like a lubricant. really wet grass is very difficult to cut, because it's heavier, and really wet grass tends to clump up inside the deck before discharge...this additional weight makes it very difficult for the mower blades to cut them down to small pieces and discharge evenly. As it clumps up, the mower blades have to deal with that and also cut grass that continues feeding under the deck and you travel...in really dry and really wet (when there are not other options to delay mowing, I slow down, AND I cut at a higher height.) planning the right conditions to mow isn't always possible, but if the choice can be made, it's best to cut when grass is not too tall and just slightly moist.
j. always check the belt routing and make sure the pulleys are not heavily varnished/glazed. And make sure the belt tensioner pulley functions properly. the pulley tensioner sometimes gets stuck in a certain range. it must have a full range sweep under spring pressure and spring back (dampen) quickly. sometimes the PTO clutch slips. It's rare to see them fail but when they do the belt will not be powered under a load adequately and the belt can slip. If you have gone through everything I mentioned above and you still cannot get the grass mowed cleanly, then take the time to test the PTO clutch. The electrical solenoid might test good. It's the clutch "pack" that might be slipping. This isn't an easy thing to test. The shadetree method is to carefully listen to the blades under power under load. If you can hear them slowing down under normal cutting resistance...then you have some clues to look at the PTO clutch and clutch pack.