Ok, there might be a couple reasons here:
Is kerning/lig/alt enabled in the app?
Is kerning activated with the font's values in the text preferences?
Is there a previous version of the font that is interfering? Learn more here.
Are there too many kerning pairs which might prevent some to be displayed? Learn more here.
The bad news is that about every app out there has its own level of implementation of type features. And accessing there features can be pretty different across apps, so let's dive a few ones from Adobe, Sketch, Apple or Microsoft.
Adobe apps (Illustrator, Photoshop, InDesign)
Check you app version
Photoshop CC 2018 and older do not yet show alternate glyphs in the quick selection previews, leaving empty spots. You can notify the Photoshop team about this issue here and help solve this bug by upvoting it ;)
Check your type options
If spacing or kerning doesn't show up correctly in Photoshop, check here whether you should disable the System Layout text option.
Ensure Kerning is set to Auto in the Character panel:
Fontself Maker
Preview issue when kerning alternates
Due to a bug in Chromium (which runs extensions on Adobe's apps), kerning applied to alternate glyphs will not show up in the main preview of Fontself Maker. In the Advanced views, the kerning will however show up ok as we developed our own text rendering engine for this part of the UI, and the kerning will also be ok when used in other apps.
Editing kerning is slow, freezes or keeps spinning
Bohemian (Sketch)
To view the proper kerning pairs you set in Fontself, make sure to enable the setting Use Default in the Text kerning options:
Apple (Pages, Keynote, TextEdit, iBooks Author, etc.)
By default, Apple apps apply kerning but may not display your extra glyph features. But you can precisely apply (or remove) them via the Typography panel:
Go to Format > Font > Show Fonts or press Command-T. Then click the gear icon (circled) and choose Typography.
Then, select the text you want to edit (the Typography panel is context sensitive, based on the selected font).
Depending on the features included in your font, you will ben able to switch each feature individually.
Also note that you can set the ligatures setting via the Advanced Options in the Format > Font Style panel.
Microsoft (Word, Powerpoint, Excel)
Kerning, ligatures & alternates are not activated by default, they must be enabled manually, and while they are supported in more recent versions of MS apps, alternates will work in Word 2011 but may not work in older versions, like Word 2008.
⚠️In recent Word versions (including Office 365), ligatures & kerning do not always work right away: try to enter some text and activate these feature from a new empty document to ensure they are REALLY not working (yep, Microsoft, please fix these decade-long issues...).
Mind that in older versions of Word, enabling ligatures could prevent kerning to be displayed (sigh... :(
If kerning or ligatures are not visible
Select your text then right-click and pick Font... (or press Cmd+D)
Check the Enable all ligatures in document
In Character Spacing check Kerning for fonts
Enter a value that is smaller that the current text size
Also, due to bugs in previous version of Fontself Maker (2.3.1 and older) such features could not work as expected. Just open them in Fontself Maker 2.3.2 or newer and Save them again.
Preview issue when kerning alternates
Due to a bug in Chromium (which runs extensions on Adobe's apps), kerning applied to alternate glyphs will not show up in the main preview of Fontself Maker. In the Advanced views, the kerning will however show up ok as we developed our own text rendering engine for this part of the UI, and the kerning will also be ok when used in other apps.