At that size and age, growth should be noticeable. However, five weeks is a relatively short period of time. There are two obvious causes that come to mind- insufficient feeding, or parasites. Parasites are uncommon in captive bred geckos, but in wild caught geckos, they are a given. If your gecko is wild caught, have your vet run a fecal.
If your gecko is captive bred, try feeding it more frequently.