Fishbone's post is informative for someone who claims to be "not a turtle expert". 
Yes, turtles aren't exactly known for growing fast. They are well known for their long life span. Some species may remain small after decades or even hundreds of years. Without growing much within a year shouldn't be a concern.
It might be helpful if you post a photo here for people to identify the species. 
What have you been feeding it? It can also be an important fact in the growth rate.
| 
