It could be a problem with the nozzle. We’re not on mains gas at home so I have a couple of Safefill bottles for the kitchen hob. Normally get them refilled at a local gas centre but on a couple of occasions have taken one to Morrisons ( who are happy to let you refill on the forecourt). On the first visit, I connected up and it took nearly ten minutes to fill. On the second occasion the same thing happened, but whilst still pressing the fill button I fiddled with the nozzle to check it was connected OK on the bottle - at which point the fill rate went back to normal. As soon as I let go it went down again. I found that if I pressed the nozzle down at a certain angle the fill went normally. Never had this problem at the gas centre (or with the Gaslow in my van) so I reckon there’s a particular problem with the nozzles at Morrisons (Bristol, Cribbs Causeway).