The Nuffield Foundation’s report How do shortages of maths teachers affect within school allocation of maths teachers to pupils?
states that shortages lead to disadvantaged secondary pupils
having less access to experienced and well-qualified teachers.

Along with this:

• inexperienced teachers or those who do not have a degree in maths, are more likely to teach Year 7 to Year 9 pupils than older year groups
• 19% of KS 3 maths teachers are inexperienced. This early exposure to inexperienced maths teachers risks
  switching pupils off maths at an age when they are forming attitudes to subjects and future choices.

They found that where maths teacher shortages occur, schools have hardly any job applications for vacant posts. Add to this that many  applicants are not suitable and you can see why they frequently have to readvertise. This can lead to schools  making  appointments that are “less than ideal.”

It is suggested that well qualified maths graduates go for better paid jobs in the private sector when the economy is doing comparatively well.