There's probably a good reason why she's reluctant to sleep inside the coop where it's safe. How long has this been going on? If she's been doing this all her life, then it's a bad habit and she needs to be coop trained by shutting her up inside for several days until she imprints on it once again.
If it's a recent development, then something is happening at roosting time where she is fearful of taking her chances in the coop. Roosting time can be rather competitive and there are sometimes conflicts where hens will knock others off the perch or peck at them unmercifully when they try to roost.
The best thing you can do is gather her up with the others and herd them into the coop early enough where everyone can still see what they're doing, including you. You will be able to stand and observe their actions while they roost and see what happens to the reluctant hen.
From what you observe, you may be able to take steps to make roosting more comfortable for the reluctant hen, and perhaps others as well.