Why might powerpoint not let me adjust the height of a table row?

The length of the row is based on the font size accordingly. Change the font size, and you will be able to adjust the height.

Thought I would add that I was stuck with this as well. What got me was that all my cells with content were size 10, but an empty cell had a font size of 18 and even though contained no content, was preventing the row from resizing.

This has happened to me. This especially happens when copying a table from MS Word to PowerPoint. Before, I have found the solution, I had dropped and dragged the border of the row to change the height with no success. The solution is very simple. You need to do a small investigation looking for white spaces inside your tables' cells. After you ensure the white spaces of the entire row cells are removed, try to change the height of the row by dropping and dragging or through the height text box in the Table Tools tabs. You should not have any problem then.