Do you have a sim that can go and stay on a lot where they should be spawning and see if they respawn at 5am game time while you are actually there on that lot?
I am (for example) having the problem where lots are not ... maintaining themselves in my absence (trash/food for example) but also that mineral spawners are now seeming to take forever to respawn. They do eventually respawn but I have to spend a long time on the lot or visit it many times (the equivalent to spending a long time on the lot) for them to spawn.
Do any spawners exist in the neighborhood area that your home/residence is? For example I have 4-5 mineral spawns near my home and they are there once a day. When I go to other areas, they are not there because I have harvested it clean -- but if I stay all day fishing and wait until 5am sometimes the minerals will respawn.
