The 22-year-old Oxford graduate took over as the programme's numbers and letters girl last month following Vorderman's departure. She was joined by sports presenter Jeff Stelling, who replaced main host Des O'Connor.
Insiders have claimed that Riley was unable to solve the show's number puzzles as quickly and accurately as Vorderman in her first three weeks of filming.
One source told the News of the World: "It's clear she hasn't managed to match Carol's skills yet. If the contestants couldn't solve a problem Carol would quickly do it for them. But during the filming for her first 15 episodes the new girl made some mistakes."
An audience member added: "It was obvious she was doing her best but she was off target a few times when I was watching. It was a little embarrassing but I'm sure she will find her feet. It can be very intimidating standing in front of a studio audience."
Vorderman announced that she had quit the Channel 4 game show in July. She later claimed that she had been "forced out" by the programme's bosses.
A Channel 4 spokeswoman insisted: "We have pre-recorded a small number of episodes so far. Things have been going very well and Rachel and Jeff make a very good team."