as with all fish, when newly introduced they might be shy or have become very timid.
if they wont take to frozen foods straight away try giving them live foods first, this usaully helps a fish to settle in and can then be put onto frozen foods without a fuss.
i had 4 dwarf puffers and 2 figure8 puffers.
they wer all fed on live bloodworms and live blackworms first and then they took happily to frozen foods within a few weeks. even brineshrimp.