It seems odd that something like the tradition for techeles could have been lost. It seems unlikely that Jews just stopped using techeles at some point and then immediately forgot. Are there no ...
The Tur writes (Orah Haim 11) that since in his days they didn't have Techelet they didn't have to be Makpid on the Huliot (the Rambam doesn't agree). Therefor the Posekim hold (Rosh, Tur, SH"A etc.) ...