CA27-29 nothing on PET

[concernedperson]

Good morning,
I was wondering if anyone had any words of wisdome or experiance.
My mother has Stage iv and has been on chemo for two years now plus herceptin.
She has been considered stable for about 6 months now but her tumor markers have steadily been rising. She got a PET scan but nothing showed up. Nothing shrunk, nothging grew.
So they are keeping her on the same chemo for right now and changing herceptin from every three weeks to every week. Does increasing the time you get herceptin make a difference? Also I know a lot of docs do not go by tumor markers, but if the numbers keep rising doesnt that usually mean something is going on maybe it just cant be seen yet?
Thank you for any advice.