Sad to say I have twisted many bass strings.  The last Steinert
I did had new bass strings put on about five years (could have been
less) before I started the restoration.  I called the supply company
and asked if their new bass strings were wrapped around a hexagon
core as described in their catalog.  I was assured they were.

I sent in the old ones and lo and behold they weren't using a hex core
-- the new cores are round and they hammer flat the wrapping around the
core before winding.  When they finish the wrapping at the other end of
the string they hammer it flat.  The resulting mess is that no amount of
turning will give a round cored string a bite on the core.

Your best bet is to find a string maker that still wraps the old way.
The hex core will then bite into the core when twisted.  If you find
a supplier let us all know so we can escape the cheapo!

Thanks
Dell Urry