If your humidity is low the rest of the time a quick spike may not have been enough to give a good shed. If your humidity is good the entire time a spike in humidity shouldn't actually be needed. It is just to help the process but not necessary. For most of my collection I do nothing different at the start of a shed and they all shed perfectly. Many times I have missed the first signs and just come home to a shed skin with no idea it was coming.

Either your humidity has been low for to long before the shed or the device you are using to measure it is off.