Harrad, who is out of contract in the summer, is Sampson's top January transfer target but has his sights set on playing at a higher level than League Two.
"It's probably a long shot now, which is a shame because he's a good player. He works hard and he gets in behind a lot," Sampson told the Northampton Chronicle.
"He thinks he's good enough for League One or the Championship but as far as I know they haven't had any takers from that level. We're the only ones that have made a bid so far.
"He and his agent haven't ruled us out and that hasn't changed but we're not going to wait forever; if he doesn't want to come here, he doesn't want to come here.
"If something else comes up in the meantime then we'll have to explore our options."
Source: PA
Source: PA