using an average summary field calculation

I have a couple of DBs - one is a price file, the other is an assessment file. So, in the price file I have product A price $X, B price $Y, C price $Z. I have created a summary average field which averages the values of X, Y and Z, say $BB in one instance. Depending how I find my records it correctly calculates the average for the records found.

In my other file I have various products attached to a region - China, Scotland and Canada, say. Under each I have different products: A, B C etc. Each product can only exist in one country.

In the product file I have imported a related field which is the price.

I have produced a report sorted by country which shows the following:

Country: China.

Product A. Price $X

Product B, Price $Y

Product C, Price $Z

Country Canada

Product D. Price $P

Product E. Price $F

and so on.

What I want to do is include a field which averages the price of all the products for one country e.g. Country China. Av. price for all products = $BB.

However, when I include the summary field above sometimes it appears, if for example, there is only one product in one country, but mostly it is blank, other times a wrong average appears when I have, say 10 products but not all have prices yet, perhaps only 5 have a price and the others are blank.

Where am I going wrong? Many thx.