Do the bricks give a signal, in an air test, once you have them out of the ground? Do you re-check the hole after removing the brick, to see if anything else was beneath (or beside) it?
If you're not getting a signal in an air test on the brick (indicating that something metal is in it), and you're no longer getting a signal out of the hole, then I think you've either got your sensitivity too high (thus blarring off the mineral content of the brick?) and you need to turn your sens. down. Oftentimes a hot-rock or cinder or something that gives a signal in the ground, ceases to give a signal (or at least, ceases to give an apparent easy signal) when doing an air test on the same object. The composure of the fixed settled setting, matrix of ground, moisture, etc.... can give a different signal in the air. Either way, this would mean your sens. is too high.
Another possibility is you're swinging wrong. The 250 is a motion (albeit sloooww motion) machine. What you classify as a "strong signal" could be a lousy signal to a proficient user, when swung right, isolated right, criss-crossed right, etc... I have seen this many times, when working with newbies, who insist they're hearing a signal, and chase it all over the field. But odd explanations come easily to life when I cross-check them for those persons. Anything as simple as the metal toe in their boots, a pipe going up the corner of the building (and they didn't know the front tip of their coil was sensitive), or they're trying to "stop" on the target, thus allowing it to "dissapear" (d/t lack of motion), or them trying to pinpoint the re-bound sounds of a larger target, etc.. etc... etc...
Your best bet is to hook up with a proficient 250 user in your area. Compare and trade off on signals, watch what they're listening for, pinpointing, etc... Because things like "sound" can not be described in printed text. It has to be shown and heard.
