What is happening is the brass you are using has a thin wall. The neck of the brass where the bullet seats is too thin for that brass.
I use a neck sizer with replaceable bushing for BR shooting. On new .223 brass I use .246 neck bushing and after I shoot it approximately 8 times then I have to use a .243 neck bushing to get the correct neck tension to seat the bullet. When the case "lengthens" and must be trimmed down. The "lengthening" is because the neck wall stretching causing the case to thin.
Since you have a Full length sizer, you do not have the option of using neck bushings. So you cannot do any adjustments on your part. If they are RCBS dies, then they are within spec and it is not the dies that you are having problems with.
Long story short.
Use different brass or a better quality.
I like winchester with has a .010 neck thickness when new. or even better use Hornady brass. it has an average neck thickness of .014 which will hold the bullet more firmly and will not allow the bullet to slip. plus you will be able to get more reloads with hornady brass.