Why is FTP blocked?

[#1]

Why is FTP sometimes blocked in the evenings?  This has been much more common in the past week or two (coinciding with my broadband approaching modem speeds).
I read somewhere in the support bit that traffic shaping may be used (and by all accounts is being used), but don't recall anything about services being restricted during busy times.  I use FTP to upload stuff to websites - this isn't an optional or 'nice to have' protocol for me unfortunately.
Any help appreciated.

[#2]

What product are you on and what's your average monthly usage?

[#3]

Hi there,
I've sent you a PM, with a little more information.
Let me know if I can be of any further help.

[#4]

Bob and Mand - Thanks for your help. 
Mand pointed out that I'd exceeded my allowance this month, and that BBYW 2 would be more suitable.  It was a bit difficult to judge my usage as the peak times have changed between this and my current product, but I think she's right and I've just asked to change over in the control panel. 
I'm going to assume this was the reason for my FTP problems.  (Possibly should have checked this earlier actually...)
Thanks again.

[#5]

Hi there,
It will have been.  Once you reach your allowance, file sharing protocols and external FTP are blocked, with other traffic being restricted to 128Kbps.
Glad all has now been sorted out