I use most of the defaults, but under Destruction set all 'false' to 'true' (i leave enhanced single target as false then toggle it with hotkey - in semi-auto mode). Change AoE count to 3. Disable burning rush unless it's a fight where you rely on quick movement.
As for not casting; post a log.
I had same problem with standing and not doing anything for some time as destro and then again start and then again stop... Point is that nothing can be seen in logs, everything looks completely normal.
What happens:
After checking everything many times I have found out that Pure for some reason is changing Throttle time from 0,5 to 5 in General setting on its own and when this happens.
When it happens:
Most certain way to see it is to restart HB after one run and start it again. load Pure, open config and there it is 5 instead of 0,5.
Current solution:
Check it and change it whenever you enter instance. I personally delete all pure settings every time I enter instance, start new HB and quickly change settings to what I want them to be as I found it most reliable.
Because nothing else is getting changed in routine this is most definitely what is causing problem and with throttle time 0,5 it will top dps meter while with 5 it will do like 1/3 of normal dps. Pure really rocks as destro when this problem doesn't occur.
Just to be safe before posting I have checked it again and here is exactly what I have done so you can reproduce it:
1. Joined lfr
2. Start HB and load pure
3. Finish lfr and exit
4. Restart HB and load Pure
After that when you open config it will be 5 in Throttle time instead of 0,5