my phone has problem with space

I bought Sony Xperia c at truworths in 2014, it still on 2 years guarantee. It has problem with space, every time I want to download or update it says insufficient storage but it has 16GB on it, I've changed the settings to SD and let the system decide but is still saying insufficient storage. On March 2016 I returned the phone to truworths and get it back in first week of April 2016, hoping the problem is solved, but its still giving me the same problem again. Please help