Thanks for the additional cultural details.
BTW, none of the media ingredients I use or recommend are anything other than 100% mineral, other than those I suggest as amendments to the media, such as dilute fertilizer solutions, dried insect powder, or Trichoderma harzianum inoculant.
My favorite media are all-mineral ingredients:Media: 1 part Schultz aquatic plant soil (sintered fullers earth) and 1 part 20 or 30 grit silica sand --- pre-moisten the sand (not wet, just damp) and blend a very small amount of iron oxide powder into the sand before mixing all ingredients together, I mix this media thoroughly while lightly spraying with a 40ppm solution of 20-20-20 soluble fertilizer with trace elements (which is absorbed/adsorbed by the aquatic plant soil and fortifies it).
Amendments to media: dried, powdered, insects with a small amount of Trichoderma harzianum inoculum mixed in.
After reading the additional details of your cultural conditions, it seems to me that the missing factor is most definitely a lack of nutrients. Yes, successfully fertilizing Pinguicula can be tricky and, if done improperly, can cause the loss of the plant, but the same can happen if essential nutrition is withheld, it just may linger a little longer.