How do I use purchased ringtones as text alert tones?

Have a question for experienced iOS technical types. I've purchased several ringtones from iTunes and have used them on my iPhones (5/5S/6). I'm able to assign all of them as ringtones both as defaults and assigned to individual contacts. I want to also use them for text alert tones for some individual contacts as well. When I try that, not all of them show in the list so that I can assign them. When I try to select my default Text Tone, not all of them show up there, only about 1/2 of them. I've even purchased a text tone for a contact (off iTunes), and it stayed there for only two days, then became just another ringtone. Any suggestions?

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