I believe Sora shifters, like all shimano "STI" type shifters are subject to that cheap grease they use turning hard and seizing springs and levers.
I recently had some older 200GS shifters from a MTB that required quite a bit of TLC, gunbore cleaner, Teflon spray oil and patience to get them loose and working correctly.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk