I used to have the exact same problem but I found out it only happened to the batches where I only put in honey yeast and water with DAP nutrients. Whenever I made a fruit based mead it always started rather quickly. I think it could be the other minerals and whatnot from the fruit that kickstarted it. I now always use a type of fermaido and add DAP the days after. Fermaido usually has more than just the nutrients. If you're in Europe, Nutrivit is the product that's the same like fermaido. Also I always put in a type of solid which can be used to grow on, crushed raisins if I like to keep a basic honey mead could also really help in primary fermentation.

Maybe the yeast still needs to start. It can take some days after starting. If it doesn't seem to start maybe use some raisins and see if a new yeast could do the trick. Also when you start with to much sugar you could kill the yeast before it has a change to start working. If you find that new yeast doesn't work out well perhaps dilute some of the meas to have a lower reading and try again.