Cloning in winter is hard you say? Maybe you're trying a bit too hard. They just take a bit longer to get there at this time of year.
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12 x Charlotte's Angels on the left. 24 x Leda Uno on the right.
Simple coco/perlite mix. No magic juices except for a quick dunk in cloning gel & straight into the cups a week ago.
No heating (I'm on top of a mountain in Vic.) & no artificial lighting (shed skylights only).
All achieved with a smashed collarbone - one-handed.
I'm sure that not all of them will work-out, but that's par for the course with clones (I tried using a DIY aerocloner a couple of times & composted everything as horrible failures).
Just give them time. Don't over-think this stuff mate!