Stretch Across all Devices not Working

[idle_fret]

Have chosen the above. When I preview page it fits my screen, blinks and then narrows down considerably. What am I doing wrong? Web Designer Premium 19. Thanks in advance.

[Acorn]

The 'blink' is it resizing to fit your page to the browser width.
If this is excessive then you probably have an object that is far wider than your page size.
In your design zoom out to 25% and Select All.
If the bounding box is off the page, this is your issue.

The second common fault is incorrect coding in your Website, Page or Placeholder code boxes.
Clone your design and delete any added code, box by box and Preview until you are sizing correctly.
The prior deletion is where the problem sits.

Another might be too small a value in the Scale to Fit Width setting, if you have such set or available.

Failing all these, upload your design to TG here.

Acorn