[MrB (Simon)]

Collimation, dew, vibrations, thin high cloud, quality of optics (chromatic aberration, wavefront error, scatter etc), guiding errors, tube currents, warm mirror, just to name a few.

But seeing(depending on fl) and focus (and maybe collimation for reflectors) would have to be the most common.

Dew is easy to overlook, heavy dew is easily noticed on lenses and in images, but light dew can be barely seen on a lens/mirror but can easily bloat brighter stars. Same goes for very thin cloud at a dark site, almost invisible by eye but will bloat stars.