We asked a stamp in the church of Tui last year on a sundaymorning before the Mass service together with some other pilgrims. The man in charge was very upset and anoid about our question ! Maybe too busy and the service was more important than the sello in the credential at that moment. So maybe this can happen to you in Porto
More sello adventures
In Ordes last month (camino Inglès ) our sello was refused by a priest I asked for. Ordes was off the camino (3 kms !) in his opinion so no sellos given ! We slept in Ordes due to lack of an albergue around ! Maybe this priest slept badly the night before .!
The hotel we stayed was pleased to stamp our credentials
We made a touristical detour from Barcelos to Braga and got sellos in our credentials of the church bom Jesus do Monte. No problem !
Last month nobody complained at the pilgrims office that our first stamp was from the Sé cathedral in Lisbon while we started 800 kms northwards in Ferrol (so the second sello was from the Ferrol tourist office )
Bom caminho and good luck on sunday morning