I presume you purchased these caps over the Internet ? Did you purchase them all from reputable dealers ? I have to say , purchasing anything unseen , online , is fraught with problems - there are so many fakes out there .
My advice ? Purchase from a dealer , at a show , if possible , so you can see the piece and get a feel for it . Failing that , go to a reputable dealer , not someone on eBay . It will cost you more , but you're more likely to get an original .
Finally , fabrics varied greatly in the 30s and 40s , and toward the end of the war , quality suffered . It may be that the cap you bought was made from an inferior fabric , and comparing it to your zelts is maybe unhelpful . Plus things deteriorate according to how they're stored . After 70+ years , your cap may be suffering the ravages of time .
Hope this helps ,
Jäger Stefan Halle 3./I./Geb.Jag.Reg.100