Price not changing for Canada

Hi, I have 4 currencies: dollar, pound, euro and canadian dollar.
USA and Canada have the same price, but the currency is different (US$ and $)
I use fixed price option and “Change currency follow” billing address.
In checkout page, if I set country to Spain the price is correctly set to euro, if I set country to UK it’s pound, and USA dollar, but if I chose Canada the currency and price remain both the previous one selected.

Example:

country=US -> price=US$99.99 OK
then I change country to Canada: price=US$99.99 should be $99.99
then I change country to Spain: price=€109.99 OK
I change back to Canada: price=€109.99 NOT OK

The second issue is about the GET parameter ?wmc-currency
In product page I can the parameter (i.e. ?wmc-currency=EUR), but that parameter disappears in checkout page.
I need that parameter because I have external landing pages that link directly to my checkout, how can I solve it?
I tried to paste the parameter directly in the checkout url, but the page started to loop for ever.
Thank you in advance, Stefania