2nd season but no more grass?

Greetings all. First off let me say I am totally new to hay having been a city boy up til now so please bear with me.

My folks purchased a property and last year had about 5 acres of plot converted from the corn/soybean rotation to a hay mixture of Timothy grass and alfalfa mix. I believe the initial seeding was 80/20 grass/alf. The field was rebalanced and tweaked (I don't know the precise details) for the hay mix and was prepped and planted. I should add that we have sandy low quality soil out here. Anyways, due to weather conditions we had our first cut in late summer which was of ok quality for a new field I'm told despite the weeds. No further cuts were done that season.

Fast forward to today, and we've got half the field a beautiful flowing green mass of nearly all alf with little weeding and little grass. The other half where the alf took is growing beautifully and spreading, but there are large open areas of dead grass that are sitting there growing nothing but the occasional small weed here and there.

So now that history is finished,I ask can someone provide me with some background info and educated guesses/hunches as to the problem (if there is one)? Is the grass gone for good or just dormant? Should we attempt to replant some grass or just let the field become an alf field and reseed some alf?

I'm not expecting complete solutions, but any info that can be provided would be greatly appreciated as at this point as currently I don't know enough to even know what I need to ask.

Again, thanks for the help.

2nd season but no more grass?