This is probably a well-known issue at this point, but troop movements are incredibly buggy. The slider for selecting what number of troops to send often freezes (bar will be white as though full, but number on the left will be zero; number on right often does not reflect actual number of available troops), which requires you to cancel and try again. While troops are moving from your army to a territory or vice versa, recruitment can behave in weird ways, often either stopping before you hit the unit cap, or else creating troops in excess of the unit cap. Sometimes, troops sent to a region just never get there. I also once got a glitch where I could not station troops in Marbia anymore. I think this was cause by a raid destroying the garrison, but for some reason it didn't let me build a new one afterwards, so I had no choice but to reload an earlier save.
Speaking of which, it's a small gripe I have with this game that, immediately after conquering a territory, you can get raided in that territory, before having the chance to station any troops to defend it, which is both a cheap shot and doesn't make much sense in my opinion. I think after conquering a territory, at least part of the standing army you used to conquer it should be automatically assigned there; in war, even after the capital falls and you've technically taken over, not only is your army still there from having just conquered the region, but you'd also need to keep that army around for a while to stamp any lingering resistance while people get used to the new regime.