Basically, this problem can occur in case you have incorrectly configured the splitter and its sizes, e.g the sum of the heights of all the RadPanes is bigger than the height of the splitter itself. In case you have set size equal to 100% for the panes, please remove it because in this case if the first pane should be 100%, there is no space left for the others.
In case you cannot find the incorrect size and you need further assistance, please either fix the provided live url (I am getting an error when loading the page - see the attached screenshot for your reference) or best - prepare a sample, fully runnable reproduction demo, open a new support ticket and send it to me along with detailed reproduction steps and explanations. Once I am able to examine the setup, I will help you fix the problem.
the Telerik team