Dear MR Javi,
1’45” @ 21C should easily give you a second yellow color if you have enough iodine in your box.
2nd yellow should be OK but Pobboravsky has shown that thicker 110uM plate coatings may be as much as 16 times less sensitive to light (4stops) than thinner 30uM coatings.
Just to be sure that this is not the problem, you should try masking off part of the plate with paper and fuming portions of the plate for 40” to magenta, and for 80“ to light blue and then for 105” to second yellow..
You don’t mention the aperture you are using but even 20min at f/8 might not be enough if the light is less than 13EV. If you have an exposure meter, use it! Despite claims that Dags are only sensitive to UV, Pobboravsky has shown that Dags are also sensitive to light up to blue 450uM so a meter reading is a good start.
Lastly the red plastic may not be as good as Rubylith for developing Becquerel Dags.
You don’t say whether your Dags have no image particles, which indicates underexposure, or that they have a whitish coating, which indicates overall fog.