Customers can't get to sites set up as cnames

[Kelly Witherbee]

Hello,

I'm a newbie DNS person looking for some help.  We have a web-based product
that we set up for customers and the URL for each individual customer is
customer.domain.ext.  These are set up as cnames to our main site,
www.recruitingcenter.net.  In the past, our new customers have had no
problems getting to their sites.  Starting recently, they've had
intermittent problems accessing these "virtual" domains.  We have probably
close to 100 cnames set up under the recruitingcenter.net domain.  Old
customers don't have any problems (example:
techpartners.recruitingcenter.net), but new customers can *sometimes* see
their sites and sometimes not (example:  flextime.recruitingcenter.net).  Of
course, customers on AOL can't get to our newest ones...but that's really no
big shocker.  I am guessing that the problem is that their ISPs haven't
cleared their DNS cache in a while, but the thing that gets me is that we
have never had this problem in the past.  Do we have too many cnames set up?
Help!

Thanks,
Kelly