Firstly ensure that it is the phantom power and not something else that is causing the problem. Check that the mic and mic cable is not damaged. Use different mics that require phantom power to eliminate the possibility of a broken mic. Also, use a mic that does not require phantom power to make sure that the problem is not elsewhere in the unit.
If you have checked the above and it still seems as if the phantom power is playing up then we advise returning the unit to the store if it is still under the one year warranty or, if not, contacting the distributor for your country to organise repair (all contact details can be found on the distributors page).