This is known as ‘God’s little joke’ (not really I made that up) and it’s very simply that you can’t get a comfortable angle or reach quite deep enough to finger yourself to orgasm. Your arms just aren’t long enough and neither are your fingers to do it at the right depth, or hard enough.
See the difference…(source @thesecretroom )
The simple solution is a dildo! A nice bendy one designed hit your g-spot like I recommend in my toys page
An alternative though… is self-fisting. Seriously, If you can manage it, you can get the pressure from your whole hand that just your own fingers can’t.
You can do it from the front:
Or the back:
But you better be bendy and have small hands…