Hi everyone

I am new to the forum but have been playing STW for a long time and just recently started on MTW VI.
I do not have RTW simply because my old timer PC cannot handle it. Alas, one day

I will prop. have a few questions for stuff I cant find on the forums (no search function yet).

So here's the first one:
I am playing the Danes and have all of Britain/Ireland in my possesion. I keep these territories with a minimum of forces whilst I keep a strong fleet in the Channel. I'm thinking that it's impossible for an enemy to move a force through the Channel waters while I have my fleet in it.
I thought so because that is how it works for me when someone puts a boat there I cant move back and forth.

But to my suprise the Spanish were able to invade Wessex by putting a single boat in the Channel while my fleet was still in it.
Why does the blockade only work one way?
Does it have anything to do with how fast the ships are?
My fleet consists of Caravels and Longboats.

Please