I would expect there to be some variation between samples. But if they are propagated the same way I would think the titers should be somewhat similar (maybe within 5-fold).

Changes in phage titer are possible:

- if you do not carefully follow the MOI for phage propagation. You can try standardising the bacterial and phage culture parameters more accurately by tracking the bacterial inoculum by OD and measuring the amount of phage to be added.

- possible influence of internal evolution of the strain (e.g. loss of individual plasmids affecting the biochemistry of the bacterium)

- some bacteriophages do not grow well and sustainably in plastic vessels. Try growing them in glass.