Fountain pen filling + ink question

[Burlington]

I was wondering if it’s me or the pen, but I’ve never been able to draw up ink through the nib with my platinum Izumo

Instead I need to dismantle and place the end of the converter cartridge in the ink and draw it up directly into the tube.

I figure it could be two things if it’s down to user error - either the nib isn’t submerged enough so have tested it all the way in or there isn’t a strong enough vacuum from the piston converter to draw the ink upwards against gravity.

Should all FPs fill via the nib or do some just not work that way ?

The other thing I’ve noticed is how much better (smoother and wetter) the platinum writes with black waterman ink I tried.

With diamine inks before it had a tendency to be a bit scratchy and dry if I wrote too fast.

After not using it so much during the last 2 years due to various reasons, the platinum with the waterman ink is now my go to favourite each day.

Are there any recommendations please for ‘wet’ fast flowing inks to try with the platinum pen/nib ?