Until it's fixed, try going forward to the next week (futilely) and then hitting refresh. When I do that, it does update to the future week.
Under review
I am still looking for a solution but I cannot reproduce the issue on any of my internet browsers (chrome, firefox, internet explorer, safari) so it makes it difficult to chasethe issue. In the meantime, please follow Jacquelyn's advice and refresh the page after hitting the next week button. (Thanks Jacquelyn)
I have the same problem  (Firefox 34.05 for Mac, german version). My solution for the meantime is to go somewhere in april and then back to the wanted week appears. I hope in april the problem is solved anyway. 
Refreshing is the easiest way.
Is anyone of you using WeekPlan using an application shortcut? or bookmark?