DMR fills a bulk tunnel to 2 to 2.2 metres, so depth alone creates three different environments, and the far end rarely matches the door end. Take from front, middle and back, at top, middle and bottom, before calling a sample representative.
Timing matters as much as position. Moisture and ammonia both change while the mass is cooling, so a sample taken at noon and one taken at midnight answer different questions. Fix the stage at which you sample, write it down, and keep it the same for every batch so the numbers can be compared.