Shift lever loose on rear derailleur shift

I have Shimano 105 flight deck shift levers, mated to Shimano 105 rear
derailleur on a touring bike. Recently, when pushing the right hand (rear
derailleur) shift lever to the left in order to shift from a smaller to a
larger rear sprocket it often requires two pushes - the first push is very
loose and then it finally catches and increases tension.

It seems to me this might be due to a gummed up or sticky rear derailleur
cable but I'm not sure. Has anyone else experienced this and know a fix?

Larry Gagnon
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