What Avast version are you using ?
Whilst I'm still using Avast Free 22.5.xxxx I'm not experiencing this issue, automatic viruses definition updates appear to be working fine with self-defence on.
Have you tried an Avast repair ?
There was another topic relating to virus definitions being a problem, but I can't seem to find it, I think bob3160 posted in it (about some sort of out of sequence VPS update causing a problem). I think he also posted a reference to a resolution.