Using refractometer for final gravity

If I take a sample of fermenting wort to figure out the current gravity I find that I cannot get a defined reading. By this I mean reading the Brix scale is difficult because it is blurry.
Is this because the amount of sugars in the wort have been converted into alcohol?
Has anyone else found this?

Cheers and bollocks
TDA

I'm just guessing here but sometimes it helps if you degass it.  Fermenting wort has quite a bit of CO2 in it.

Put a bit in a small glass and stir it for a while and then let it sit for a bit before you measure it.

Cheers!
Brad

Ha! Good onya TDA! That was my question for the day  

Thanks Brad.

Cheers
Steve