Why is my mobile hotspot not enabling

[dcdeans]

I have a samsung S4 mini, running Android 4.4.2 on giffgaff network in the UK. Starting about a month ago, my Mobile hotspot will not fully enable. When I switch it on, the symbol goes partially green and says "Enabling Mobile hotspot.....". But it never actually fully enables, it just stays like that.

Does anyone know why this might be?

Cheers,
David