There are two types of allotment types, one tracks the number of cases opened per contract and the other tracks by total minutes spent on activities for the case. First, verify that the contract template used to create the contract has the correct allotment type. If not, you will need to cancel the contract and create a new one that uses a contract template with the correct allotment type.
This issue can also occur if the contract was created from the related records for an account. When the contract is created, the system can associate only one record with the contract, which is the account. The contract template is lost. If this occurs, recreate the contract from the Contracts list in the Service area, and then reassociate any cases or activities.