I found that connecting more than two extra devices to the hub can cause problems.
The only way around this is to make sure you purchase a hub that is 'powered' ie, you can plug it into the mains using its own power cord.
Once the voltage supplied from your laptop to the hub goes over a critical level (too many devices attached) you'll find some work sometimes and not others or they don't work properly. Scanning devices seem to be the worst offenders.
Anyway, that's my experience with USB extension hubs.