You are lucky in that yours are already halter trained and therefore have been "civilised" as much as a Dexter can be.
I would imagine that you only ever attempt to handle them when you want to catch them. You are therefore training them to move away whenever you approach.
If you want to be able to catch an animal at will, you need to build up a relationship with it, not just go and grab it when it suits you.
So - forget catching them and concentrate on handling them. Cattle love being scratched on the backside, so whenever you get near, eg when they're eating, give them a friendly scratch near the tail and then walk on. Gradually they'll get used to this and realise you aren't trying to grab them whenever you approach. Then, when they'll happily stand for you to scratch them, move your hand along their back. A stiff brush is handy here, and they love being brushed along their back and sides. Gradually progress forward - under the neck is another favoured spot, and soon you'll be able to touch the halter with no problem. But do not grab it. Just gently clip on the lead rope and apply pressure so the cow knows she's caught and won't run off with you. Always use a lead rope on a cow, never just hold the halter unless it's an emergency. Once a cow has nearly run off with your fingers trapped between the head collar and her head you will understand why.
This all takes time. You won't achieve it in a day. But once your cows enjoy you coming up to scratch and brush you then they wil actually start approaching you and you'll have no trouble catching them again.
I have my dexters grazing on a kilometre long stretch of river bank, but every day when I check them, I give their back a little scratch and talk to them. So when I want to catch one up for any reason, they have no problem with me just putting a rope halter on and tying them up.