I think once you get the correct nozzle, then your problems will mostly disappear. I don't have any 0402 components on hand, but I am willing to bet that the 503 nozzle is too big. Typical spacing between terminations on 0402 components is 0.7mm, A 503 nozzle is 1mm outside diameter. The terminations on those resistor look rather poor (ie, lots of variation in tinning / solder dimension), so if the nozzle cant seat properly in the flat middle section of the component, it will make a poor vacuum seal, sit cocked on the nozzle, and will likely shift during high speed movements. Just my thoughts....
Thanks though for doing all this testing. I have not had the time to do much work with mine, but I did manage to get things working really well during my initial test, to be confident in its operation, however, I only use 805 and larger comps at the moment, so a 503 nozzle is more than adequate for my needs. I am looking forward to seeing how the 502 nozzle works for your 0402 components.