cache fills very slowly

I have the ExpressCache in 3 different machines.  On my fastest machine, the cache fills quickly to over 20GB, but then slows to 26GB never reaching 29GB.  On the slower machines, it takes a week or more to get to 15GB.  This is using 1.3.  Ideas?

the cache is only going to fuill based of LBA usage. Are you actually using all of these machines frequently enough to cache that much data? if you are not reading LBAs frequently there will be nothing for it to cache except the boot files.