Ig replacement therapy should protect against the common circulating bacteria and viruses but some common viral infections do not generate good lasting protective antibodies or these are not helpful in preventing recurrence, for example of the common cold. Patients with underlying lung, ear or sinus disease (e.g. bronchiectasis) may still get infection from infectious reservoirs in those tissues.  Occasionally trough Ig levels dip for a variety of reasons. If the infections are needing further antibiotic therapy this should be discussed with your immunology team.