How long does Symparica Trio take to work? Dog still has fleas 3 months later.

Small toy breed.

Not really exposed to other dogs or animals.

No carpet in house - all solid flooring with no rugs. Dog bedding regularly hot washed.

He developed fleas with symptoms over 3 months ago. Did a one spot which resolved symptoms after 2 weeks.

Has been back on regular Symparica Trio for 2 months now. So in total 3 months of regular flea treatment.

I found another live flea in his fur today.

I thought this oral treatment was meant to be effective? What more can or should be be doing?