Can you share with us the specific schedule of your care (just so we can help you out with suggestions)? From the photos, the tree appears dead. But I am confused by its appearance. If you look closely at the stage of annual development, you will see a lot of secondary buds that are about to break into candles, as well as some secondary candles that have just opened and died. This looks like a tree frozen in time during the late summer. Did you candle prune it last year? If I had to guess, it died last Fall and just slowly dried out over the winter. I am 98% sure it is dead, and I have never seen a JBP push buds when the needles are all dead, but the true indicator is the tree - have you seen ANY movement in those buds in the last month? (expanding/elongating?)
This will sound a little strange since I normally would never say this about JBP, but an outside windowsill might be one location where your trees might need a little protection from the elements - particularly the desiccating effects of wind and sun. With an open bonsai soil mix and planted in a shallow bonsai pot, sitting out on a windowsill in the wind and sun your soil could easily dry out in a couple of hours if you aren't careful.
