formula error! Format issue with formula!

I made some test and each time the field is a formula field, it stay undefined... it must be a number field or other format... Sound weird...why we could not take formula field in equation ?

web browser... 

Its like: field A = number + number 2

            field B = field A + number

it s like the field B wont take the formula because it s formula sources... It s still undefined... How can I correct it?

I made it work but it s incredibly slow and I have a good connection...

but that was just a test field...here the real one...it s a pop-up

let a := concat("Montant donné diffère du montant de la facture de: ", abs(sum(number(Montant), number(Montant_2), number(Montant_3)) - sum(total_vente, prix_shipping, taxes)));
if abs(sum(number(Montant), number(Montant_2), number(Montant_3)) - sum(total_vente, prix_shipping, taxes)) > 0.01 then
alert(a)
end

it have works for a time but now it wont...but I lost some field...some other wont refresh...my 'show only if' formula comes to issue too...

something seem to be wrong with the connection I think...

is there a way to check connection quality? I lost all data from my table...it start spinning and sometimes I have to quit and come back to finaly load data...and after few minutes if I change table it start spinning again...and data desapear...

my connection here is gigabite and every other site works well... What can i do?

Hope the picture would be fine...but that wath I experiment since few days...my formula wont update and data desapear... any ideas??