@thewonglv’s steps work – Make sure you don’t have heights set on the slider itself, the mask, and the slides in it (you do have heights currently on all three, randomly across various viewports), also, take the 5px padding off of the bottom of the slider that you have set on the portrait phone viewport.
Glad it worked.
That preview link doesn’t work fyi
To add a new blank slide select the slider, go to the settings tab, click new slide (as shown below)
To duplicate an existing slide, select the one you want to duplicate from the navigator tab and press the duplicate button at the bottom of the screen (as shown below)