Answer:  The most common reason for this happening is something bridging the gap between the post and the "valve". In between the post and the valves on the EVI are small plastic insulators. These prevent conduction between the post and the "valve". If conduction occurs several things can happen such as cancellation of the "valve" and post, or more commonly the post having no effect. This can also be caused by improper adjustment of the touch-sensitivity screw on the bottom of the instrument (see the Q&A regarding that phenomenon). If anything such as moisture in this case, dirt, a hair, etc. bridges the "valve" and the post it will cause problems. To prevent this clean the area with alcohol and a swab or spray it with an instant drying "Zero Residue Cleaner" available at Radio Shack.