Just be patient, if you can't get near him/her just go into the field and sit down (perhaps take a book) and wait, then when s/he comes over to you give them a treat then just put headcollar on for a few steps then treat and take it off and increase the steps each day.
Mine has a habit of planting once the headcollar is on, so I go for getting a step in any direction. That gets them out of the mindset of 'I'm standing here and not moving'
Charlie can be a pain to catch,food sometimes works but not always, a quiet sensitive approach mostly with food works with him mostly. Currently he wears a headcollar in field which is much easier! Lol but only cause there's leccy fence all round x