What causes positive and negative deviations when weighing analytical samples?
There can be plenty. There is no definite list. I would suggest to look up the following terms in Skoog or Daniel Harris' Analytical Chemistry. Worth a trip to the library now :-) The following concepts give rise to systematic errors

(i) buoyancy error
(ii) temperature differences 
(iii) electrostatic charge build up on the sample
(v) moisture
(iv) above all loss of calibration of the balance.