Apossible reason is that if new fasteners were added as a part of the sub-floor prep for the new floor, a new nail could have caught a joist edge, split it and not be in full contact with the wood.
- As you walk across the floor, the boards move against each other or slide against nail shafts to produce the squeaks.
- If this is the case, the new flooring could be disassembled and the fastener removed or secured.
Also squeaks and gaps can result from too soft an underlayment, an un-flat subfloor or improper acclimation - taking the new flooring from a semi-conditioned storage area and installing it without waiting for the flooring to adjust to the humidity levels in your home.
Additionally, be sure there is a perimeter expansion gap around the room and any pipe penetrations, such as a floor radiator would require. Some manufacturers require the expansion gap width match the flooring thickness.
For more information:
Read "I am installing hardwood flooring and putting Aluma-Foil underneath. Will the floors squeak?" a Q&A answered by Erik Henry.