I haven't parked at SFO in a while so dunno if this is still the case, but for a while at both long term and short term (which also accepts prepaid via QR code) there was someone stationed at the entrances to help people hold their phones at just the right distance/angle to be read... IIRC there is sort of a plate/icon indicating where to hold your phone, and I found that actually holding it just beneath that was what usually worked for me.