I am not sure what's went wrong with it. But there are things you have to do for newly planted grass:

1. water daily especially in the morning and evening. Not noon as evaporation will remove most water out of it.

2. Too much fertilizer kills the grass! To know if you use too much or too little, try fertilized a portion of it and see if this part is growing better than the rest. Also, heavy rain washes away the fertilizer! When there are too much fertilizer it will "burn" the grass and turn yellowish. Moist or water the grass first before fertilizer and make sure is well spread and then water a little more so that the fertilizer get into the ground than on top of the grass leaves.

3. Planting. first you wet the ground, flood it real wet but do not disturb the sand, then put the new grass on top press it down. That is the way to do and do not water till next day. This is the way to plant new grass. When water be very careful not to hose it, use shower lightly. Hosing it is a mistake. By doing so you wash away the mud/soil of the grass root down to the sands. When the root hold no soil it get sick and die! That is why you must water it with light shower. By pressing it down to the sand is to make sure there is not too much void or big air pocket and good contact between the newly plant grass root and the sands.