How can I fix my Baxi Solo 24 HE boiler?

The burner often takes two attempts to fire up.It is still heating water & rads,but something is wrong & I want to nip it in the bud before a complete failure occurs!
The burner will ignite first time when boiler is cold,but this problem only occurs when the boiler is warm,& not all the time,although instances are becoming more frequent.
Would appreciate some feedback please?
Many thanks