Digital BW, The Print

I've just reprinted an image that I originally used as a test several 
weeks ago to determine what papers I should stock. My two favorite test 
prints, on Hahnemule Photo Rag 308 and William Turner, have been 
leaning against some books for approx 2 weeks, and upon doing the 
reprint, (on Photo Rag 308) I noticed a considerable difference in 
colour between the new print and the older 2 prints. The new print was 
considerably colder in tone, and in a side by side comparison, one 
would almost call the older prints brownish in tone by comparison.

I have not changed my ink, MIS FS through a CIS on a 1200, and I am 
wondering if this ink / paper combination  takes time to 'cure'. By 
that I mean that the print has a colour shift as it dries and ages. If 
this is the case, does anyone know how long this process takes. I've 
been looking at these prints for a day now, and there is still a 
noticable difference in tones.

If 'curing' is not an issue, can anyone suggest what is happening?

Wolf Kutnahorsky