Our central air conditioning unit continuously runs and our thermostat is set for 76degrees and it won't go below 77 degrees. Why is that?

Ac isn`t putting out cold enough air.

A few things to check/do yourself is
1. make sure the filter is clean
2. make sure the condenser coils are clean (wash out real good from the inside out) with power turned off to unit
3. make sure the evaporater coil is clean ( Lowe's sells a coil cleaner for this purpose) (again make sure clean on the inside of coil)
4. make sure all return air vents are open and not covered
5. check all supply ducts and make sure you`re not losing cool air where its not needed
This will give you a good head start
You could be low on refrigerant,but these items will need to be checked out first,, you`ll have to call out the hvac pro`s to do the refrigerant