The Terriers remain in seventh place, three points behind Colchester in sixth, with only eight games left of a League One campaign which is in danger of fizzling out.
Saturday's frustrating 1-1 draw means Town have won only once in six matches and have not beaten any team in the top six all season.
Clark, whose side have beaten most of the division's supposedly weaker opponents at home, fears they are making a habit out of freezing on the big stage.
He said: "I thought the players looked nervous. I can't understand why. We've got to be brave enough to play in front of our own fans and deal with the expectations. It should be a dream to come to a stadium like this and play in front of big crowds. They've handled the expectation before, but for some reason it has become a burden.
"Maybe it was just a bad day at the office. Thankfully, the play-off situation has not changed. Colchester were three points ahead before the game and that's still the case."
Jordan Rhodes put Town in front with a looping header on 53 minutes but Charlton's Kyel Reid, on loan from Sheffield United, cancelled out the 20th goal of Rhodes's season with a blistering drive into the bottom right corner five minutes later.
Huddersfield Town: Smithies; Peltier, P Clarke, Trotman, Heffernan; Pilkington, Collins, Drinkwater, Roberts; Robinson, Rhodes. Unused substitutes: Eastwood, N Clarke, T Clarke, Williams, Berrett, Pearce, Hunt.
Charlton Athletic: Randolf; Richardson, S Sodje, Dailly, Barrowdale (Solly 82); Wagstaff, Racon, Semedo, Reid; Forster (A Sodje 85), Mooney. Unused substitutes: Warner, Sam, Llera, Shelvy, Spring.
Referee: Mr Jarnail Singh (Middlesex)
Man of the match: Jordan Rhodes.
Follow all the goals go in with our Match Centre service.
This season the Yorkshire Post has an exciting new match-day service, called Match Centre. Here you will find minute-by-minute updates, stats and team formations. Plus reports from every game in England, from the Premier League down to the Blue Square Premier.
Simply select your game from the pull down menu, then click on the 'i' information button on the right of the fixture and it will give you access to everything you need to know about your team.
So if you can't make it to a game, head for the Match Centre and we will give you the best seat in the house. Click the link below.
For the latest team news, score and commentary from your team check out our new Match Centre