Your last comment is incorrect, of course you can image without plan mode, using autorun mode which has been present since version 1 of the asiair.
You dont need autoguiding with it either, just use your mounts in built sidereal tracking, but autoguiding will be more accurate as long as your PA is, and if youve calibrated the guide camera it will start autoguiding when you start autorun, it will also dither if set.
You haven't mentioned whether youre using filters as this has a bearing, you can set centre exposure time under one of the menus, by default it think it's 2s but you can go up to 9 I think which is needed especially when using an o3 filter, a longer center exposure time will enable the stars to saturate better and not be mistaken for noise. If you're using an uncooled camera the noise WILL get mistaken for some stars and plate solving will take a while if it even plate solves at all.
With all mounts I've used using plan mode or even the preview mode which also centres and plate solves the airs usually solve within 2 or 3 attempts, any longer and there's usually something wrong.
Cloud will obviously affect plate solving, if the cameras imaged during passing cloud it will likely fail solving and attempt again.
Imaging bright nebula in centre of frame works perfectly fine, otherwise they wouldn't being the product to market, something else is wrong with what you're doing.
8 inch you're imaging quite a medium to long focal length, so star saturation is critical as they'll be less stars in the FOV especially when paired with that camera, you also need to make sure you're using a coma corrector as coma'd stars will not solve.
