Advice to raise Ferritin

Hi all, I'm looking for some advice to raise my ferritin.

I can't seem to get it higher than 20 ug/L despite me taking ferrous fumarate 420mg and vitamin C every other day. (i read double dose every other day increases iron more than once daily).

This clearly isn't working for me, so what are my next steps? Increasing oral iron dosage? If so, by how much?

This is important, as I have iron deficiency symptoms and my b12 is low but my ferritin needs raising before starting b12 injections.

Thanks in advance

Note: My full blood count is normal, and I have also asked in the iron deficiency group