The Pompey boss said that his team did not keep the ball well enough after the break and has vowed to address their inability to kill teams off.
"Our first-half performance was decent enough, but in the second half it was backs to the wall and we didn't keep the ball well enough with their pressure eventually telling," PA quotes Whittingham as saying.
"In the first half we created a lot of good chances, but we don't seem to keep playing the football we are capable of.
"We've had a discussion about it, with calm heads and we have to make sure we get it right. Three times we've been winning away and not been able to hold out."