After bolstering their forward options with the signing of Jay Bothroyd on a free transfer this week, Rangers are keen to further strengthen up front ahead of their return to the Premier League this season.
He told Sky Sports News: "I think the clubs have agreed but I don't think we can agree with his advisors (on) certain fees.
"I'm afraid that's life as a manager nowadays. It's very difficult to actually get things over the line. I leave that to other people but it's not happened.
"I think at the moment it's on ice. I'm looking around all over as well. We've got one or two irons in the fire. "
Even if a Campbell deal fails to materialise, Warnock believes he has a quality striker at his disposal in new-boy Bothroyd.
"To get Jay in on a Bosman is really a good bit of business for us," he added. "He's a top striker.
"I didn't think we'd get him, I thought one of the bigger clubs would snap him up. I've pestered him over the past six weeks, him and his missus, and he's caved in.
"I do think its right for him. I think he's got to be playing week in, week out rather than be in a squad. I don't see anything better than him in the Premier League and we're really, really pleased to have him."