What field type are you using for your durations? Duration? Number? Text? 

Maybe some filter is enabled? 
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Hello, thanks very much for your time and help. It's a lookup field that looks up a number field. 

Hello !

No its not the case, the result is the same with a formula SUM field. any other idea ?

Did you try using a rollup field on your project records to look at the numbers in your rollup field on your tasks records? That should work. 

Hello, not sure if i understood correctly, see screenshot of the rollup formatting : 

- Table projects (Routes) : rollup of the lookup fields of field ''objectifs temps' in table tasks (visits)

- Table tasks (visits) : lookup from a linked field (order_ID) to get the ''objectif temps''

"Objectif temps(min)/adresse" is a string, which may be the reason why the total fails.
If it is a number, it will be displayed on the right side of the cell.

How about changing the Lookup field to a rollup field and using the formula Value(values) to convert the values to a number field?

Try using a rollup instead of a lookup in your table tasks.

Hello !

Thanks for the suggestion, just tried it but it returns ERROR message, see screenshot

 Tried so but still not working 😕😕 It seems the issues comes from the fact that the original field is a lookup (rollup from a lookup). Is it possible that a number data stored in a lookup field changes to a string ? How to change it back to a number ?

Oops, value(values) is an error.
Try sum(values)

By using a rollup field instead of a lookup field for that original field. 

Oh OK guys i think i almost got it ! it seems airtable rollup field switched to 0 the data when the lookup field has multiple values, see screenshot attached 

@Databaser is right, the original Lookup field also needs to be a Rollup field.