NFO won’t darken in sun

[327fed]

I use double shoulders plain ( I think they are called natural). Then I darken in sun with 1 to 2 coats NFO. Recently (different projects from same hide) NFO treated projects do not darken in sun when previous ones did darken well. I wait for a bright sunlight day. I am gonna try dying but would like to know what is going on. Thanks in advance. 

[tsunkasapa]

I think that is because the UV index isn't as high this time year. It will 'suntan', but it will take longer.

[327fed]

Seems like in about 2 hours it darkens about all it will with each coat of oil. Then in about 3 years it gets really dark. I was making my brother a holster for his birthday ( yesterday) and was gonna ship it tomorrow.  Be a few more days I guess. 

[327fed]

Would not darken in sun so I died it, 50 50 Feblings and alcohol. Streaked it. Tried to cover with shoe polish. Streaked it more. Made another today from same hide. Has some pink sections and zebra streaks that I tried to cut around. Oil and sun streaked this time. Will seal with 50-50 mop and Glo and give up.