The Highlanders fell 30-16 to the Sharks at Carisbrook, and with the loss slip to 12th on the Super 14 ladder, with one win from six games.
It was a game which has become almost a Highlanders hallmark - plenty of possession and endeavour, domination for long periods, but at the final whistle coming up short.
The home side could only muster one try, through outstanding winger Ben Smith, and went close another couple of times but tended to lack composure at critical moments.
Fullback Israel Dagg made a couple of fine breaks but could not finish, while Michael Hobbs also looked classy throughout.
Cowan, who threw out a couple of massive backhand passes during the game, said it was not a case of not being ruthless enough but just not getting the rewards they were looking for.
"It was hard, as we were all over them in the first 20 minutes, yet all we came away with it was six points," Cowan said.
"We played a lot of territory and possession in that first half but just could not get rewards for it.
They put on a good defence and we were struggling at times in attack."
Cowan said the side came back well in the second half after being 23-9 down and he thought leaking points in the start of the second half may have been because of the travel back from South Africa.
"We got a bit loose in the tackling and they started really well in that second half. But you have to give our guys some credit for we came back well."
He said the set piece had gone well, but the side just had to keep working hard.
"This is a tough competition, where we've got to keep boxing on week in week out. We're up against the Lions next, who are another tough South African side. But we need to keep believing in ourselves and our systems."
Highlanders coach Glenn Moore said it was frustrating the side had the ascendancy in the first half but could not come away with more points.
"We had a bit of ball, stole a bit of theirs and we made a lot of play but there wasn't a lot of reward. There was opportunity there to score two or three tries," Moore said.
Moore felt the try awarded to Sharks hooker Bismarck du Plessis was "tight" but said though the Sharks had lost five games they were a strong defensive side which had not had conceded many tries.
The Sharks scored three tries to one in the win and when they scored their second try by winger Stefan Terblanche after the Highlanders ran out of numbers, it looked like it was all over for the home team.
But the Highlanders came back into the game when Smith dotted down after a good blindside move from a scrum.
That left the Highlanders seven points behind with 12 minutes left but it could not quite get up with Fetu'u Vainikolo pushed out in the corner.
Sharks midfielder Adrian Jacobs broke the home side's spirits with two minutes to go when he intercepted a pass thrown by Jayden Hayward.
Others to impress for the home side were No 8 Adam Thomson and flanker Alando Soakai, while Jason Rutledge got around the field well.
The best for the visitors was first try-scorer Bismarck du Plessis, No 8 Ryan Kankowski and Terblanche.
Waitaki Boys High School beat Dunstan High School 24-7 in the final of the pre-season sevens tournament in the curtain-raiser at Carisbrook.
