Warren Gatland was at a loss to explain Wales' lacklustre first-half showing in a 25-21 defeat to England which ended their hopes of winning the Six Nations.
England controlled the first half at Twickenham on Saturday and should have had more than the one try from Anthony Watson, but 11 points from the boot of Owen Farrell put them 16-0 up at the break.
Dan Biggar gave Wales hope and Gatland's men threatened to pull off an unlikely comeback when George North and Taulupe Faletau went over late on after Farrell had taken his tally with the boot to 20 points.
England held on for a deserved victory which sealed the Triple Crown and ensured they will win a first Grand Slam since 2003 if they beat France in Paris next Saturday.
And Gatland was bemused by his side's tepid first-half performance.
He told ITV Sport: "I'm very disappointed with the first half. I said to them at half-time that we needed some energy. We made some changes, players come off the bench and gave us some impetus. We finished strongly and can only wonder what could have been.
"I don't know what the answer is [to their poor first-half performance], we can only answer that ourselves. We've got to be honest and say where is our energy? We looked tired and were not enthusiastic enough.
"I'm proud of the way the boys didn't give up. We outscored them three tries to one, but Farrell's kicks made the difference. England competed hard and they deserved to win.
"Hats off to them, they played particularly well in the first half, so full credit to them."