Potatoes in bags: Update

Here is another update on the growing potatoes in bags experiment.  So far so good!  Some of them are already growing plants about 6 inches high!  Some have only a few leaves sprouting.  This is confusing when in one bag there is a tall sprout and a short sprout.  Should I still cover them with dirt at the same time, even though they are different heights,  or wait until all have risen to 6 inches high?  I think I will cover them with dirt all at once in one bag and then wait with the others.  Even the plastic bag which was starting to look like nothing was growing in it except a weed seedling is now sporting a leafy spot.  Perhaps even the plastic bags work.  We shall see as time progresses.