David Connolly's second-half double at Fratton Park lifted Pompey off the bottom of the table at the expense of the Shakers, who are eight points adrift of safety with 10 games remaining.
Shirtliff said: "We have to win five or six of our last games but the players have to show they want it and they didn't.
"The annoying thing is we dominated this game in the first half and if we had won here it would set us up for three homes games on the trot.
"I thought in the second half we were appalling, on par with our worst this season.
"There are still 30 points to play for so we will not give up."