I've had my anycubic kossel for a good while now, and whenever I print something, it seems like it falls out of calibration very quickly. The biggest example is that the prints tend to scale up as the print progresses, or maybe the print starts to drift, and comes out slanted. This seems to never happen in any similarly priced XYZ printers. Why is this? Do XYZ printers just have an inherent advantage over deltas? Perhaps Deltas have some precision loss?
user77232's points are valid but I have done several things to mitigate these issues on my Anycubic Deltas.
First, both of my deltas have linear slide bearings. If your's has the bearing trucks that run in the extrusion slots this will lead to less precise operation and also is a source of wear over time.
Second, be sure that the end stop micro switches are positioned precisely and that their fasteners are tight. You may also want to use a mild strength Loctite on their fasteners. When operated at high extrusion rates all components are subject to significant vibration.
Third, I found that the universal joints are also a significant source of play. I remedied this to some degree by placing rubber bands across the arms at both ends so that the play was minimized. The rubber bands should be wrapped fairly tightly to perform this function.
Forth, I try to tune the belt tension so that all of the belts have the same note when plucked. There are smart phone apps that will help do this. I believe that one that I use is from Gates, a premium manufacturer of drive belts. Also, over time belts stretch so you will need to retension them periodically.
Fifth, run through the firmware calibration process regularly for both positional accuracy and for filament extrusion and retraction variables.
I don't use my deltas much any more since purchasing a popular XYZ type printer but it cost me three times what they did and it still has issues from time to time.
(The XYZ Printers are called Cartesian Printers)
Delta printers are harder to get right, because they require precision parts. The arms have to be EXACTLY the same length, the frame must be square, the universal joints must have no slop. You should check to see if any of the universal joints need replacing, and that the length of the arms are equal.
Deltas however are some of the fastest robotic platforms due to the low mass of the end effector.