Pirate, this can be so worrying and undermining :(
Lots of threads about this on MN, so have a browse :)
The commonest reasons for v. slow weight gain:
i) baby is just 'made' that way
ii) baby needs more milk - this is not the same as 'increase supply' because that is only half the story....the baby needs to get more milk inside her
You can address ii) by increasing the frequency with which you feed . It's not that uncommon for babies like yours to be feeding about 6-8 times in 24 hours, and that should be doubled so there are frequent feeds, day and night. In addition, the baby should have at least both breasts each time (google 'switch nursing' as well). Breast compression (google) is also very useful.
If you give more details about how often and what happens when your baby feeds, it'll help.
You can also call a bf helpline for a chance to discuss what to do. Your baby's feeding should be observed, if poss, and any anomalies (like tongue tie) checked for.
Mostly though, simply ensuring your baby feeds lots is the basic 'fix' - this increases your supply and at the same time gets more milk into her.
What has your HV said?
Hope this helps.