Hi
With that description ("
arrowhead" or seagull like stars) and especially if these defects are towards the edges/corners then what you probably have is a mixture of coma & astigmatism, which can manifest in what some call saggital coma flare, as shown in the diagram attached although in much greater detail than often seen with images of stars). As to the cause, you'd have to experiment a little; with/without reducer and also check that the collimation is as good as possible so as not to create a false impression re the amount of astigmatism, etc... Also Check if anything is loose or has play and whether that affects the outcome image.
https://www.handprint.com/ASTRO/IMG/comaflare.gif
Best
JA