Two first-half goals at Tynecastle left Hearts, given a 15-point penalty for entering administration, on -2 points, 19 behind 11th-placed Ross County.
"It was a sore one to take," said Locke.
"I've said to the players they can't play for 45 minutes - they have to play for 90 minutes."
Highlights - Hearts 0-2 Partick Thistle
He added: "If we do that, we give ourselves a better chance in the game - but we only showed up after half time."
The defeat by Thistle now means Hearts have not won a match in any competition since 9 November, and the spectre of relegation to the Championship continues to loom large for the cash-strapped Edinburgh side.
Locke refuses to accept that the club will be demoted at the end of the season, until survival is no longer possible for his players.
"We've just got to do better, be more aggressive and we've got to be defending better than that," said Locke.
"We're not thinking of relegation until it's mathematically impossible.
"We're up against it, there's no doubt about that - the gap is getting bigger - but especially at Tynecastle, we need to do better.
"I'd love to put my finger on it - we seem to have players who do better away from home than we do here.
"You've got to be brave when you play at home, get on the ball - and in the first half when we had the ball there wasn't enough options - as I said, we have to do it for 90 minutes."
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