Why won't my iphone and ipad connect with bluetooth?

I have used my iphone as a personal hotspot for my mum's ipad so I know it is possible to do this, but for some reason my iphone and my own ipad can't even see each other - why not and how do I resolve this? I know bluetooth doesn't have a very long range but before anyone suggests that might be an issue I sat the phone on top of the ipad and they still couldn't 'see' each other.
I have a brand new ipad and my iphone software is up to date so I don't think that can be the problem.

Use a third party app available in app store called 'Bump'

The estimate is a system problem.

My iPhone will not Bluetooth with my iPad. It has in the past but for some reason they lost each other and now will not re-connect

Why won't iPad and iPhone link up with Bluetooth?