Junk Mail doesn't seem to clear 'on schedule'

I'm using Mozilla 1.7.12, and I find the junk mail filters to be far superior to anything I was ever able to construct with Thunderbird.

One problem I've noticed - and it's not a terrible one - is that, although I've never changed the defaults in Junk Mail Controls, it's only sporadic that the junk mail is cleared from that folder in a timely fashion. The rule is that the mail is supposed to be deleted from the Junk Mail folder after 5 days, but I notice that it doesn't always clear them out. Even manually applying the junk mail controls doesn't work en masse. I can certainly do it myself, but I'm just wondering what could be preventing Mozilla from catching these more reliably.

The other question that occurs to me is whether manually clearing out the Junk Mail folder affects the filters that Mozilla is developing to catch further instances.

 

Filters are activated first, so don't worry about anything done with the Junk Controls. In my experience, filters will work alike, if set up identically.
As for Junk Controls, I have no idea what triggers their emptying. I, clearly, remember that they never seemed to emptt in Mozilla. But, usually, they will after 20 minutes or so with Mail open. In my hands, Mozilla seemed to be the slowest. And, the Junk folders were given quite a few breaks, like having a dozen of account, three of which getting all the junk. One of the accounts was my primary ISP account, and over 80% of the Spam was intercepted before the mail was downloaded.
I cut down the days all the way to 1. And, I still see most the Junk unmarked.
So, I have them go to Trash. It is not much junk, so, I have no qualms in deleting, and, don't forget to compact the Inbox, or the Junk folders after emptying. I don't use Moz1.7.12 as much as I did (mainly to check on questions like yours), but, I think that I am the best judge of what is acceptable, and, what is not. And, I had to get another primary ISP account. Now, things are peaceful.