Albion climbed to third place in the Premier League in December but have only won two of the last 11 games in all competitions since then.
Clarke is hopeful midfielder Claudio Yacob and strikers Shane Long and Marc-Antoine Fortune could return for Saturday's West Midlands derby with Aston Villa at The Hawthorns.
The Baggies boss said: "The squad has been stretched to the limit over the last six weeks. The boys that have gone in have done their best. They've tried hard, they've worked.
"Maybe the little bit of luck we were carrying a few weeks ago has deserted us a little bit.
"But all we can do is knuckle down and get on with it. As a group we know what we're trying to do.
"It's a long season and we're going to have moments when things don't go particularly well for you. This is one of those moments.
"We need to get a little bit of rhythm and continuity back into the group. That's the main aim I've got for the next three to four weeks.
"We got Peter Odemwingie and Steven Reid back on the pitch for a few minutes against QPR. Claudio Yacob has trained for a couple of days after his hamstring problem with the group and he's probably 50-50 for Saturday.
Albion defender Gareth McAuley has also called on West Brom to bring an end to their five-match winless run in all competitions.
"We've lost a few on the bounce now so we've got to dig deep," he said.
"The senior guys in the squad have to get everyone together and get everyone positive because we have a very important game on Saturday.
"Villa will be desperate for the points but we're desperate for a performance, for a win and for a clean sheet - for anything at the minute - and there's no better place to do it."