Poetry not updating sub dependencies despite version constraint allowing it

My code is using a package (taskiq), which in turn depends on another package (pycron). Why is poetry update not updating pycron despite version constraints allowing it?

$ poetry update
Updating dependencies
Resolving dependencies... (2.2s)

No dependencies to install or update

$ poetry show -o                                                  
pycron    3.0.0  3.1.1  Simple cron-like parser, which determines if current datetime matches conditions.

$ poetry show --why --tree pycron   
taskiq 0.11.10 Distributed task queue with full async support
└── pycron >=3.0.0,<4.0.0

Solution

You can answer such questions by insisting that poetry install the latest version of the project that you expect to be updated eg poetry add "pycron>=3.1.1"

Then one of three things will happen:

poetry will refuse to make that change and will tell you the conflict that prevents it from doing so
poetry will make the change: but to make that work it will have to downgrade something else. This happens when the latest versions of two packages conflict in some way, but a solution can be found by downgrading either one of them.
(very unlikely) poetry will make the change without downgrading anything else, and you will still be confused about why it did not do so in the first place