Calin,
Looking at your zone. My bet is lighting. Try keeping in a control environment like an old fish tank. I have an old 55 gal. I use to keep my Sansevieria in for the winter. Just one 4 ft. shop lite with the new T-8 bright light bulbs (2) at 16 hrs. a day is more than enough. I also use a 4 ft. mirror on the back side to increase the lighting. Also keep on the dry side. Plant in trays or in pots. Just an idea. Some use heat lamps as well but not with the cost of electricity. Cheaper to replace with new each year than the cost of keeping thru the winter. The shop light is total of 64 watts and I use the light by my chair for reading so win win. Also with tiny seedlings, they could be placed 6 inches from the lights.
Stush
