Described in this forum thread-http://www.telerik.com/community/forums/aspnet-ajax/scheduler/timeline-view-doesn-t-line-up-correctly.aspx#2764757
Related Feedback item: http://feedback.telerik.com/Project/108/Feedback/Details/88820-add-support-for-reminders-description-custom-attributes-in-radscheduler-exchang
Thus, appointment resizing is not possible in RadScheduler with RenderMode="Mobile" if a time slot is tapped before tapping the appointment. Note: If the RadScheduler is scrolled between the time slot tap and the appointment tap, then resizing is possible again.