zone transfers delay

[Paula Saito Telloli Martins]

I have problems to understand why the zone transfer of any domain is
delaying... I have implemented dynamic update and , even though, I still
have the delay. DNS spends sometimes 20 minutes and other times "50 minutes"
to make the zone transfer. It begun almost 2 months ago, suddenly,  and I
can´t find the reason.
 
Please, If someone had the same problem or has witnessed it, write me !
 
Tks in advance !
 
Paula.