TAX sometimes not added

My shop only has two zone definitions and 10 Zones

I created two accounts for myself (when I started 2 years ago) and use that to test.
I only had to add tax in September this year, the web site started in Nov 2010

If I place an order with my test account everything works fine
BUT sometimes when some clients place an order it shows "sales tax" on the form but the amount is zero.

All my products are in a tax class
Tax Rates have been set up for both zone definitions.

I then changed my "Basis of tax" for both product and shipping tax from "Shipping" to "store"
This to me would be the logical way that everybody will pay tax because tax will be calculated according to
the location of the store. BUT still with basis of tax set on store I had 3 more orders this morning without tax. Before I changed anything I first placed a test order and on my test order tax was added.

I now took the last three orders where tax were not added in in the database table "address_book" compared the entries of those addresses to the entries in my two test profiles. All of them have entries at the same place. The only entries which is different is the "entry_zone_id" I guess because they in different zones than me.

I see at all addresses the "entry_state" field is empty. I do not think that can be a problem because on my two test account those are also empty

I do not know where to look any further.
Is there any other place where I can look which will effect this

shop only has two zone definitions and 10 Zones
Is it 2 or is it 10??

Under Zones there are 10
Under Zone definitions there are 2

Previously I only had one zone
But on 15 Nov I added more zones
It seems like all the new zones I added are the ones which do not add tax.

It ads tax when I use my dummy user because my dummy user is in the zone which is working.
I now registered a new dummy user in a different zone and tested it and with that user it does not add tax.

So it has something to do with all the new zones I added.

What I can still not understand is that if I choose "basis of product tax" to be "Shop" why does it not add tax to some clients.
I mean if I tell it to calculate the basis of the tax according to the shop's address then the client's address should not have anything to do with the tax calculations.

We use the same tax all over the country and I only ship to South Africa. So I do not have to calculate different taxes for different areas.

Check location/taxes & that you have a tax rate defined for all zones

I have.
At tax rates page's zone field it only let me choose between South Africa and Namibia which is the two I have setup under "zone definition"

The different provinces I have under "Zones" is not there

Thanx Cobra you pointed me in the right direction.
I did setup the zone definition but I did not click the "details" button which takes you to another page where you have to setup the zones again, under that zone definition
Seems like that was the problem.

Glad you got it figured out