Many times I have come across questions like no emails are sent when an "item is modified" from a issue tracking list.
While troubleshooting such issues we can use below scenarios:
1. A user creates an item and assigns to self
2. A user creates an item and assigns to a different user
We need to check in which scenario the email is not sent.
If the user mentions that the email is not sent in scenario 1 then here is the possible explanation.
The logical reason for the above is just because there is no need of an email when the user knows that there is a change in the item as he has himself done those changes.
If the user mentions that the email is not sent in scenario 2 then :
In such scenarios we need check if option "Require content approval for submitted items” under Versioning settings link of the list settings is enabled.
If that option is enabled then we need to "Approve" the item. Once item is approved, immediately an email is sent to the person to whom the task is assigned . But that email is sent as a change email.
This behavior seems to be by design