Hi
There was a Google Update in March: http://moz.com/google-algorithm-change
Developers have a tend of not implementing GA properly and tracking things incorrectly - our developers do it all the time.
If your developers are saying its been tracking wrong, which channels have been affected, are these channels now up and Organic down since they made the changes.
If you have access to Search metrics - has your Search Visibility dropped over the same time period.
I would then do some further digging as Fabio touched on, and as well as what devices aren't performing as well, what pages aren't performing as well. What has changed on these pages.
Or is it a simple case - you have stayed pretty still and your competition has caught up to you and over took you?