Generally this happens when a map is 2048 or larger.
It took me forever to figure it out but had to laugh when I figured it out. Open up the attribute window and go to where you would load the texture..where the preview should be just double click it.
Not sure why they decided to change that up, kind of annoying you have to double click what ones you have to see, but that's the only way i've solved it. /shrug
Probably they´re just to big. There's a setting for "max resolution for swatches" in the display tab of window>settings/preferences>preferences. Increase that and you'll be able to see a preview.
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It took me forever to figure it out but had to laugh when I figured it out. Open up the attribute window and go to where you would load the texture..where the preview should be just double click it.
Not sure why they decided to change that up, kind of annoying you have to double click what ones you have to see, but that's the only way i've solved it. /shrug
Also I think you can force it by right-clicking on the file node in Hypershade and doing "Refresh Swatch".
Nice to have those back.