The 22-year-old was farmed out on loan to Leyton Orient earlier this season, but made a surprise return to the starting line-up against Blackpool a fortnight ago and scored on what was his first Championship start of the campaign.
Wagstaff has since started the last two matches, including the win at Blackburn and Saturday's last-minute defeat by Sheffield Wednesday.
He said: "I had an opportunity to go out on loan again but I told the gaffer I wanted to try to get into the team.
"Obviously that was quite hopeful thinking because I was quite a way down the pecking order at the time, but I've managed to work my way back in.
"I always believed I would get back in the side."