BellaOnline Forums Hobbies and Crafts Sewing hemming polyester

I am hemming(tring too) a prom dress made of polyester Itis mostly black but has two pink sections coming down the front so I'm useing invisiable thread the problem is the fabric is waving and it will not keep a steady depth inthe hem without puckering help please I'm not sure if my machine setting is incorrect or the needle,thread, me I,m at my wits end

I know there is a way of doing this on a machine, but I'd hem it by hand