Edit your plan and on the Pricing tab ensure you've "Manual renewal extends expiration date as follows:" set to "Renewal starts at expiration date or at payment date - whichever happens last (allows free gap)" as you maybe forcing your users to renew when they last expired instead of when they paid. You can correct their dates for those that are currently messed up by editing their subscription within CBSubs > Subscriptions and clicking the ID under the ID column.