Update problem with .wsp on my site

I have created a site scoped feature that will activate each time a new my site is provisioned for a user. I have done this using feature stapling.

The only content of my site scoped feature, that activates on users site collection, is an event reciever that creates some lists. The code works fine, but When I try to update the code, and build a new package and run an update command with powershell, the code is not updated when new sites are provisioned. This is a bit strange because the code that doesn't seem to get updated is in code behind.

It is not updated even if I retract the solution and re deploy the solution. If I delete the lists that where created by the feature and manually reactivate the feature then I get the updated version.

Any idea why this is happening ?