When working in photomerge in Elements 15 the hide/reveal functions are not working properly. The reveal function does not work at all and the hide function will only allow a spot at a time and will not flow as desired.
Generally users think that saying it's the same thing will give more weight to their complaint, but if it is not, chances to get an appropriate answer are lost. That's why it is recommended to start a new discussion for each different issue, stating your OS, OS version, Software version and all relevant details. For instance in this case: which Photomerge effect? However, in your case, the common factor seems like painting on a mask does not work. Both Photomerge and guided edits use masks and let you 'paint' with a brush to mask more or less. You have to be careful about the settings of the brush, normally full black by default.It's often possible to switch to 'Expert' mode to get more control over painting on the mask.