Salt licks(er.. mineral sites) work better during spring and summer with lots of green stuff for them to munch on. They do get used occasionally during fall and winter, but much less than other times of year. Back in early august, I had 1400 pics in one week on a mineral site. Around 1000 was common. Now, I'll be lucky to get 100. Just my experience.

Also, try making your own mineral sites. Cheap, easy, and works better(better nutrition, they will eat anything salty so there is not much difference in attraction for any of them) than anything I have ever seen for sale.