Potatoes require a lot of sunshine and extra lot of water. They grow underground by sending out very small threads of roots to gather nutrients and when enclosed, such as yours in a bag, they'll need an extra helping of a good, balanced fertilizer about every two weeks or so. By balanced, I mean the second and third numbers on the fertilizer package should be larger numerically than the first, such as 10-16-12 or even 10-16-16.
The watering should be consistent... enough to thoroughly reach the roots but not enough to have them standing in water which is easy to do in bags or other containers.
If your variety is an "early" or even "second early" they may be a non-flowering varietal. As mentioned, main crops usually flower.
By the way, even if the the tops turn yellow, you can leave them in the dirt to get really nice sized spuds as late as the first frost... just don't let the ground freeze. We harvest late potatoes here in the western U.S. when there are no tops remaining at all.