Once finished a painting there may arise glossy and matte or flat areas in the canvas. That looks not nice, especially exposed in wrong lighting. It is caused by a combination of the colour used, the type and amount of thinning agent added and the absorption of the ground.
The amount of oil in the paint as well as the amount of solvent that is added can vary per colour. It´s called sunken-in areas and it has nothing to do with a bad painting technique. Treating the sunken-in areas, once they are thoroughly dry to the touch, with a very thin coat of retouching varnish restores the gloss and colour.
If the sunken-in areas are very absorbent it may be necessary to repeat the procedure (once dry) in order for the gloss and colour to be of a sufficient level. The varnish dries in a few hours and leaves a porous film which in turn gives suitable adherence for a following coat of paint. It is very important to apply retouching varnish very sparingly, as the paint, not yet completely dry, may dissolve in the solvent of the varnish. Use for applying a broad varnish brush. The painting can dry now completely. After a drying time of at least one year is a layer of final varnish is recommended.