OK, the orientation of the Apps will depend on the developer, so if some show as horizontal and some show as vertical, that is how the developer has programmed them as the default view.
You need to determine if it's a Hardware or Software issue in the iPad.
You do not really have any control on the Hardware side of things so initially, so at the moment, don't worry about that.
Software, you do have some control over.
Is there a setting in an App that may be running in the background, that you have set not to rotate? If you can think of anything, go to the settings of that App and turn off the lock rotation setting and see if that resolves the issue.
Have you recently installed a new App, and since then, has the issue developed. If so, remove the new App and see if it fixes the issue.
If you cannot think of an App setting or any new Apps, then you need to consider reinstalling the whole OS to see if that resolves the issue.
If you need to to that, the first thing you need to do is to Backup the iPad to iTunes, and also you make sure you download any new photos/movies that you have on the iPad to you computer.
Then with the iPad all backed up, Restore the device to 'New' using iTunes.
This will wipe the iPad and resore it to factory settings.
Now test the iPad and see if the screen rotation issue is resolved.
If it is resolved, it was something in the iPads settings or Apps that was causing the issue.
Now you have two choices, you can reconnect to itunes and restore the iPad again, but this time from 'Backup' and it will bring back all the old settings, Apps etc onto the iPad, and test again to see if the issue is back again, or just resync the device with all the settings you want for each area in iTunes.
If none of the Software solutions work, then it is a Hardware issue and you need to take the device to Apple for them to resolve for you.
Hope this helps.
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