There are many reasons why rodents may not be attracted to a bait such as Contrac Blox. One very common reason is that the bait is not fresh; baits should be used within 1 year of opening, and stations should be changed every 30 days for the best acceptance. You should also be sure to wear gloves when servicing bait stations and handling rodenticides. This not only protects the person handling the bait and stations from contaminants and possible disease, but it also keeps human scent and oils off the bait so as not to deter the rodents from it.
It can be difficult to locate competing food sources. Mice will travel up to about 10-20 feet for food from their nesting site, while rats will travel much farther, 50-100 feet from their nesting site. Bait placements should be close enough together to allow easy access to the bait. Keep in mind also that if this is a service call, then you have not been there to monitor the sanitation all the time, so there could be trash or other food sources that have been left out. All food storage areas, food containers, areas around appliances, and trash receptacles should be checked for rodent activity.
If you have used fresh bait multiple times and are unable to locate and competing food sources but still see new rodent activity, then you may want to consider using a different bait. Just like people, rodents can prefer one food over another. Soft Baits such as First Strike have very high palatability and are great to rotate with something like Contrac.