Request assign notification not sent from time to time

Here is scenario:
request created via web with default technician group set and in most cases technician from this default group picks up request or it is assigned by manager - notifications sent without any problem.
By sometimes request is for another group and when manager assigns technician (from other than default group) SDP won't sent notification to technician (and what is strange - not always happens like that).

All notification rules seems to be fine - this another group technician is not notified about every new request - he need only notifications about reuqest assigned to him.