The biggest problem that people have with this, especially guys because of weight distribution, is balance. The swimmer gets close to actually putting their butt back into the cockpit and they capsize again doing that because they go off balance. It is not uncommon for new paddlers to go thru more than one session to get this down, because there are only so many times you can recapsize before you are too wet and cold and tired to try again.
You typically enter from over the back deck. Anything that keeps you higher from the water makes it harder to manage the balance and much easier to recapsize. That includes a higher rear deck (you'll see the Brit style boats usually have very low rear decks), seat backs that you have to climb over (hence the preference among many for back bands), crap stored on the deck, really scary looking rudders that cause you to raise yourself up to avoid getting cut... lots of stuff.
One exercise that works very, very well and is hard to convince people to do helps with the balance problem. Take the boat to shallow water, so each new capsize is not a tiring swim, and practice crawling around on the deck. Slide forward to the bow, turn around, slide to the rear, do the same thing. Sit on the boat behind or ahead of the cockpit and turn around to face the opposite way - still sitting on the deck.
It takes very little time to find out that assisted rescues are a lot easier, but even there some practice is needed to make it reliable. It only takes a moment of inattention for the rescuer to loose their grip on the swimmer's boat, or not stabilize it right and have to start all over again, or lose their own paddle.
This stuff is not hard. It DOES require practice and a willingness to get wet. Unfortunately too few people are willing to put in that time.