Personal Hotspot

Personal Hotspot can be disabled by your network, usually if this is the problem the Hotspot option is unavailable or locked so you can't use it.

The other issue I have noticed is that on my phones the hotspot usually shuts down if it is not used. To make it available I have to toggle the Hotspot Off/On again (Allow Others to Join), then it sometimes takes a few minutes before my Windows laptop sees it and connects. Strangely, my MacBook connects much more reliably than the Windows machine.