Oiling out

[Ilaria Rosselli Del Turco]

I have a portrait of a friend which I have been working on a couple of years ago. Last year I decided to repaint some areas and I oiled them out just to help me start again.
Well this year I decided to make that painting really work and I cut it and restretched on a smaller frame, but while I was doing so the areas I painted last year completely peeled off !
I was actually happy as it was where I wanted to rework anyway, so I now sanded it and have a good base to repaint, but I am worried that it could peel again.
Shall I use some retouch varnish to improve adherence ? Did the paint peel off because I oiled out (with linseed oil) a painting that was too old, so completely dry? Also if I use retouch varnish should I only put it down on the area that I intend to work on that day ( the painting is quite large and I am not going to rework the head ) ?
I hope someone can help me before I mess up again!
Ilaria