Some piggies never grow very large and very quickly. they can still live a normal life span.
My Papi (rescue born soon after arrival to a sow from a really bad background) only weighed 450g at 6 months old and was extremely slow to put on weight (she was stuck on 660g for over a month around 9 months) but eventually crept into the normal adult weight range at 800-900g. Some piggies stay even below that and stop growing sooner.
As long as your girl is a good heft (see weight guide for that), I would not worry. If the issue is genetic or congenital (i.e. your baby has been born with it), then there is nothing you can do about it.
If you are worried about her health, I would have her thoroughly vet checked.
Has she got companions that are pushing her out during feeding time? You can find tips for food bullying.
If she is an only piggy, you may want to strongly consider getting her a companion; guinea pigs are group animals and not wired to live on their own. They still have the instinct of the wild ancestor species to move to their feeding grounds twice daily in a large group and to eat together. There is nothing like the 'I want what you have' instinct to boost the appetite!
Arrival in a home from the perspective of pet shop guinea pigs
Be patient and go for quality food instead of empty fattening calories.
Weight - Monitoring and Management
Long Term Balanced General And Special Needs Guinea Pig Diets