There are several recent threads about similar problems. Sounds like you're getting ash built up and restricting air flow thru the bed. How long does the first fire burn? When does the fire start to die down? Myself included, many new burners may not shake the grates aggressively enough, allowing ash to build up and compact over time, choking the fire. Although I don't weigh my ashes, one rule of thumb I try to keep in mind is that, if I'm loading ~60 lbs of coal per day, I should be removing at least ~ 6 pounds or so of ash per day. This assumes the coal has 10% ash (which will vary somewhat with different coal brands) and NO unburnt coal. In practice, the presence of some amount of unburned coal is almost inevitable, and will increase the weight of the daily ashes. I always shake until I begin to see an orange glow under each grate coming from the fire above, not merely until I see the first few live coals drop into the pan.