thank you for your answer, 

i'm leaving the solution here in case someone else has the same issue,

the problem was that the same field service was used in more than one sales order that automatically creates a project.

so when you create a sales order you must also create a new field service for it to connect to the field service project.

thank you

IMPORTANT EDIT:

after further investigation, it was not that the field service was used more than once, but that inside the field service there was a project template set. so, you can actually use the same field service for all the sales orders, but leave the project template inside the field service blank.

thank you