Retreat

check
is your task force leader unarmed or got dont get hurt as a stratergy
is your task force leader well suplyed with both suplyes and ordinance

also seperate out any unarmed suport ships and put them into a seperate task force

there is a minor bug which causes all ships without troops on board to retreat if you order an an Assoult on a planet. They should still go in and deal with the planetery defences and only disengage when your troops are on the planet.

This will be fixed by 1.3x as its in one of the post 1.25 change logs

I don't suppose your ships are using autocannons? (in Fallen Haven's mod IRM). Regular direct-fire or torpedo weapons can destroy colonies, but autocannons cannot, that was something that threw me off when I started IRM.

Either way, check your weapons to see if they can actually damage planets, not all weapons can, so your ships would behave as if they were unarmed and retreat when they see a colony they can't hurt.

the previus post leads me to ask you if you are playing stock or a mod game

It also occurs to me have you agreed to amy treaties which stop you bombarding planets since the last war? as that might be the cause

Its the treaty unfortunately once you agree with any one not to bombard planets then you cant bombard not just their planets as you might expect but any ones at all. However it’s generally more profitable to take a planet reasonably intact than glass it from orbit so if you have the troops to hand you have nothing to lose and much to gain.

You are lucky you are playing stock as conquest is easier than hacking ones way through the defences that the balance Mod or IRM will put up mines satellites fighters drones and for major worlds squadrons of ships and bases not to mention enough weapon platforms to cut a state of the art light cruiser in half in a salvo and blow it to bits in two.

Once you land expect militia at least but more usually quite a few troops backed up by fighters and in the balance mod the weapon platforms you thought you had outflanked by landing troops

Well, it'd be kind of silly if it affected only the empire you had the treaty with, since the first thing that happens when you declare war is that all your treaties are broken.

Suction feet are not to be trifled with!