It's still likely inhibitors (or prot. K) remaining in your DNA.
You could try a thorough (but gently since the DNA should be big)
phenol (make sure your phenol is in good shape) by gently mixing in
the phenol for a long period (ie an hour). I usually do 2 phenols and
a chloroform extraction and then spool out the DNA. Some people also
diazlyze. One can even purify by Cs spins (but I haven't). There are
also companies that sell genomic kits.
You could also digest your DNA in more dilute reactions (diluting
the inhibitors). My experience, however, is for mouse DNA.
rocket1 at bu.edu