Ovlovers80

Hi, I've had a Callow for about a year now, (the 4 vent base version), but due to house move Iv'e only just got round to firing up this week, so I had a dry run/ curing/ priming two days ago, just charcoal and wood chunks, no water or meat and the temp topped at about 250c, so I was pleased, thinking I'm going to have to shut down to reduce the temperature for cooking, I then carried out a boiling water test on the thermometer and it showed spot on 100c, although I am going to get a digital thermometer, they're not easy to get at the moment, Amazon shows not currently available for the models I'm interested in, anyway today I've gone ahead and started what I planned to be an 8 hour smoke with a pork shoulder butt, the same charcoal/ wood chunk fire, and water pan full with hot water, however I'm frustrated to find the temp topped out at about 120c after about 15min, then reduced to about 105c, now 2.5hrs in and I'm struggling to maintain much above 100c, all vents fully open, I've tried allowing the fuel to reduce, tried adding more fuel but I am unable to increase the temp, I'd prefer about 125c - 130c, am I missing something, I'd welcome your suggestions/ tips, as you guys seem pretty clued up on the callow, I appreciate this is my first run and it's very much a learning curve, but I would have thought I could have achieved higher temps. Pete.