Thanks for the replys !
My bottle is a class 2- iso 3164, high density polyethylene (HDPE), then I don't really think that could be the problem, i bought that bottles in a Chemical supply store, looking good for any chemicals, even strong acids i believe.
For the temp, that's interesting because the last 2 weeks it was store at 16 Celsius, but it hapened also at 19 as well, but probably slowly.
Thanks Gerald, that must be a temperature issue, for now i will keep dissolving crystals when they appear and prepare only small quantities of solution.

other thing : the quantity of crystals is quite large, way more of what could be up the saturation threshold (if that is clear ^^), it looks like the Potassium dichromate in the solution would all go back to crystals, that is strange, I never read that anywhere.

to gmikol : i was looking for ammonium dichromate also, but it's very difficult to get, and makes expo time much higher. though I 'd like to try...