If you don’t stick to the opponent when blocking and defending, you are not in control. Either you are in close, or far away. In between, you don’t have control. So when you are close, be sure to stick. Muchimi is also related to kakie, as explained in the excerpt from an interview with Morio Higaonna Sensei that follows below. So one of the best ways to develop muchimi "stickiness" is when practicing kakie.
"When we block, we shouldn’t use force. “Muchimi” means that before your adversary attacks you already have to be in control. It also means that before the blow arrives you must read it and curb it. In this way we can take hold of the opponent’s arm and push him back. If this is done after the opponent has used force, the result is a clash of forces. We must lessen the blow before force has been applied. If we push before the blow arrives, the distance between you and your adversary is reduced. This is why the most effective blows are the short ones. In order for them to contain energy our breathing needs to be correct. The Masters of old used to teach that once we had blocked we had to pull and get in closer. This, in Okinawa, is called “Kakie”. But also in China this technique was expressed by a sound like “kaki”. And once you had gotten closer, you followed on with a technique called “Kou” (leaning). By blocking in this way, we pull the opponent and we attack by following the arm. And this is done at great speed, like a whip. If we follow the arm, it doesn’t matter how our opponent moves, we will find him."
Morio Higaonna Sensei