2010MY recurring DPF light

Hello, my 130k Outback had a forced dpf re-gen about a month ago after the DPF light started flashing. A bit over a month and 500 ish miles later its flashing again.

My work commute has changed recently from 50 miles each way Inc. 20+ miles on fast dual carriageway to about 35 min drive on rural b roads, so averaging 40 ish mph and slower through villages. Its rare that it's ever used on very short journeys.

FWIW I don't use supermarket fuel as it seemed to always run lumpy when I did previously.

Any thoughts on why the DPF would be a recurring issue I'd thought even the shorter commute would be enough for it to get hot enough to do a re-gen if needed. Its got a full Subaru dealer service history up until last year, so it's well looked after. 

Could it just be an age thing?

Thanks