A: For Sigma issue:

The Sigma EF-500 flashes have very small chromed contacts for the batteries. Even though they may not look corroded, they may not power up or conduct enough power to fully cycle up. Use a polishing tip in a dremel or an ink eraser tip to clean the internal wire contacts and the flat contacts on the door. I use dielectric grease on a q-tip to apply a coat to all contacts and cleaned battery terminals to minimise future issues.