Hi Alec,
Precision dataconverters and breadboards don't usually go well together. Lots of possibilities for poor connections and miswiring. Breadboards are also notorious for noise and oscillation.
As far as communication, you can write something wrong and read it wrong, and you may have what you think is correct but the ADS1212 can have incorrect values in the register. At the very least you should read out the registers to see if you have the default power up values.
Best regards,
Bob B
