Most often it is moisture in the flux, though it can be a sign of age as well from what I understand. However, flux formulations are what is often called in the industry 'black art' in that the mining sources (or compound sources in general) change and the formulator has to make adjustments, constant adjustments, to formulations to maintain the characteristics of the flux. And sometimes a particular characterisitic isn't quite what it was before. A flux purchased today IS NOT the same flux you purchased 5 years ago.
Another question however, since the only time I've had this same trouble consistently is with Lincoln MIL 800H. This is an excellent flux with great mechanicals but down inside a deep groove you cannot hardly beat this stuff out with a hammer. Works beautifully in thin wall shallow grooves and on caps.
So, have you changed your bevel configuration?