If you’re having troubles with controls, it’s more likely to be the controls you are using, rather than the cups you are testing!
A large part of this is due to the storage requirements for controls. Here’s an outline of what the spec sheet for the Detectabuse brand of controls says:
Stable until the expiration date when stored in the freezer (-10ºC to -20ºC) or the fridge (2ºC to 8ºC).
Stable for up to 6 months when stored in a freezer (-10ºC to -20ºC). Can be thawed/frozen up to 5 times.
Stable for 31 days when stored in the fridge (2ºC to 8ºC).
Most testers around the country are storing controls solutions in the fridge, but are unaware that the controls will become unstable after a month!