First off sweet looking game
At the very top of your script put this line math.randomseed(tick())
This will ensure that everything is 100% random everytime.
Sometimes when everything is random oddities can occur where all the deer tend to spawn in the middle, if you are sure it is 100% a problem with the spawning and not an oddity you could try creating a visualization function that puts a red part at every spawn position so you can see where all the spawns are, you could also try dividing your map up into 9 quadrants and spawning an equal amount of dear inside each of the 9 boxes to ensure they are distributed equally thru ought the map.