The Basque side ended their Heineken Cup Pool 4 campaign with a 25-16 victory over Viadana on Friday, meaning they finished second on 18 points behind Saracens.
It is highly unlikely that will be enough to see them advance into the quarter-finals though, leaving Isaac and the rest of the coaching staff to reflect on another disappointing European campaign.
Les Biarrots, who have again struggled behind the scrum this season, are hardly setting the domestic Top 14 alight either and Isaac is getting concerned.
"We feel that, among the players, there is a lot of frustration and that something isn't working," he said.
"We are asking the players to be positive and forward-thinking. So now it is up to them to get their heads in the right place.
"But we can't continue like this until the end of the season."
He added: "It is always the same story. We get into good positions but we fail to finish them off when we get in behind.
"There have again been a lot of mistakes and technical errors (against Viadana) that are unacceptable at this level. It is hard to take."