Server available to some people but not to others

Oddly enough half of my friends can’t join my gmod dedicated server. About half of them can. Same goes for the general public. The rest get ‘server not responding’
Any basic idea on how to solve this?

EDIT: Oops, I accidentally had the wrong tab open, I should have posted here:
My bad.
Mods, move at will.

Use standard HL2 (GMod) ports. These are between 27010 and 27030, I think.

I do. My port is 27015.


Is it a home hosted dedicated server, or a rented one?

Home hosted…
But only because I have fiber-optic internet.