Sometimes, when you have some fields with dynamic content in a Pentaho Report, they overlap other elements when the content overflows. That's not always (or never) the intended behavior with such fields and the solution is relatively easy.
Reports in PRD have a default "canvas" layout, in which we position the elements anywhere we want them to be, and that position we choose is respected no matter what. If you want elements to push down others in relation with their content, you should be using a "block" layout.
Note that this layout will only allow a stacked positioning, no more drag-and-place positioning but, if you still need this layout for some parts of your report, just insert a new band and assign it a "canvas" layout and start positioning elements inside this band as you like.
To change the layout of a band (including the Details band), select it using the "Structure" pane tree (or directly clicking on it), and look for the "layout" property in the "size & position" group in the Style sub pane.