What Causes The Production Of Sound At The Back Of The Tongue?

I am a speech pathologist working with an 8year old, otherwise typically developing girl. She presents with an unusual phonological process, known as backing i.e. sounds that should be produced in the front of her mouth by tongue tip for e.g. /t/, /d/ and /n/, are being produced in the back of the mouth with back of tongue elevation e.g. /k/,/g/ and /ng/. Now in a younger child this might not be so odd. This girl is quite bright and achieving above national standard academically. An oral exam revealed no real irregularities except for a very sensitive hard palate. What could be the cause of this? And should she be referred to a specialist for evaluation? I ask because more than 6mths of weekly therapy has not yielded (in my opinion) great results.