Products page not saving when price empty

I use woocommerce and avada theme. I have also added pluggings: compare, ask for a quote and fedex shipping.
I have created a list of products to sell each pages having exactly the same properties. So the Call for a Price plugin serves me when a comparison of products is made, the table will show “Ask for a price”, on all other listings it is the ask for a quote that shows (including on products page).Now that was working beautiful and the *ask for a price”pluging was working for about 20 of my products and then when I came to the last 5, suddenly I can no more save my product pages when prices are empty. I will get an error message asking to fill in a price. I can still somewhat make this work by putting a 0 value but then when I create compare tables, they will show “free” under the products and no more the “call for a price”. Any idea why call for a price worked for all products except 5 when there is absolutely no difference whatsoever between those products pages?
(I tried to clone pages too, but it just seems that now I can no more save products with empty prices on)
Thank you