Do you always use sheaves at least the diameter required by the manufacturer? Do you know that diameter off the top of your head? Does someone check the crews - at every pole or tower - to make sure they're complying?
Never use a sheave smaller than that recommended in the cable manufacturer's procedures unless you have specific approval to do so from the vendors. Big sheaves are a real hassle, so line crews will sometimes try to take shortcuts and use smaller, lighter, easier-to-rig sheaves than the ones approved by the manufacturer. Don't do this – we'd rather be your designer than your failure analysis consultant! Undersized sheaves damage cable internally in ways that may not be obvious from the outside. They void your cable warranty, too. (We've got a tech note on sheaves.)
We'll ask one more time – even if you say your installers don't use undersized sheaves -- are you sure of this?