1. Thumbnails missing ( for themes with timthumb)
Most of my new themes uses the wordpress featured thumbnail feature . The thumbnails are processed with an image cropping library called titmthumb script. In order to see the featured thumbnails you will have to make sure that all the following things are met.
Set featured thumbnail from post editor.
Make sure the image is hosted on your wordpress server. Remote linked images wont work.
Make sure your host supports PHP 5 and GD library script. ( Free hosting services may not meet this)
The cache folder in your theme should have CHMOD 755 or 777 permission
2. Thumbnails missing ( themes without timthumb )
Make sure you have set the featured thumbnail if it supports. In some cases you may see the thumbnail images appear to flat or squeezed. This is likely to happen to previously set thumbnails when you activate a new theme with a different image dimensions. You can use Regenerate Thumbnails plugins to create new set of thumbnails.
For some old themes like tribune “Custom fields” are used for thumbnails. Check the custom field key from its tutorials and use them.
3. Thumbnails missing on WordPress MU setup
If you are using my theme with timthumb script on a WPMU setup there is a likeliness that the featured thumbnails do not show up. I have explained a fix for this issue here – Read here
4. Slider not working
Featured sliders are set to work from a category. In most themes there is an option to select a category for the slider. Make sure you have at least 2 posts in the category you have set for slider. For some themes there will be an option to set the number of slides for the slider. Make sure you enter the number equal or less than that of the posts available in the slider category. For example if the featured category you selected has only 4 posts then you should only enter number of slides as 4 or less. If you enter 6 number of slides then it may break some sliders.
Most sliders are jQuery based. Some times it is possible for the slider to have conflict with your plugins that may run on some other js libraries. In this case you may de activate your plugins and iidentify the conflicting plugin and find an alternative for it.
more will be added…