First of all make sure that in your financial settings you have the provinces of all the customers you sell to as well, not just the one you sell FROM. You can do this by going to Settings > Financial and then Selecting every area that you need to activate the Sales Tax for.
Next check these two settings:
1. the zip codes when they're entered in to Allmoxy need to have the space between the two parts (T5T 3Y4 rather than T5T3Y4).
2. you'll need to go to your shipping page (Settings -> operations -> Shipping) and enter in your "ship from/will call" zip (in that same format as I mentioned) so that it knows where the order is coming from.
