Hmmm, maybe try "rehydrating" the pen.
Place the pen with enough fresh KCL calibration/storage solution in a small cup to fully engulf the glass for ~24hrs or so. But, before that, cleaning the bulb doesn't hurt. I usually use wetted q-tips to remove any gunk / film from the bulb and the junction. The KCL solution should "measure" around 4, FWIW.
Rinse then try to recalibrate again.