My Mazda 6 20004 model sometimes turns over but won't start leave it a while then it starts, not sure what is going on?

Next time it doesn't start, check for power and firing at the coil/s and injectors. Sounds like you may have corrosion at a plug or plugs. Check for codes, even if the light isn't on. Look at the history codes.

may be you should make a diagnostic and you can do it in a garage or you can do it by yourself if you want to do it by yourself you need some tools and you can find some good ones here: diyobd2.fr

It can be your mounting house for your spark plugs how to change it ?how to cheak it ? Well there should be 2 or 3 bolts holding it take the bolts out turn on your truck and remove it like you would if it was a cap in the inside should spin if doesent then thats another problem it wouldn't be your mounting house that will be a snapped little belt in the inside of motor not really sure what's it called. So back to your mounting house disconnect all ur spark plugs only connected to the mounting house ( remember we're they go) then remove the bolts and put the new one

Typically this is a sign of a faulty crankshaft position sensor.

no
resetting the ecm is with a scan tool
but you can use the scan tool to find the failed sensor preventing it from starting

A fuel pump going out can cause this problem. Have you change the fuel filter latley? As far as a miss fire. You need to find out what cylender, It could be a coil pack acting up. If it missfires you should have a code.Autozone or O'reillys will check it for free

Won't start or engine won't turn over and try to start?

key transponder ,dealer only job sorry

sounds like a faulty alternator have it checked and serviced.

I WOULD GUESS IF YOUR STARER IS KICKING IN AND THE DOESN,T START IT WILL BE YOUR TIMING BELT MAY HAVE JUMPED OR BROKE . IF THE CAR DOES THIS IT SOMETIMES WILL RUN BUT WILL SOUND LIKE IT IS GOING TO FALL APART OTHER TIMES IT JUST WON,T START. YOU CAN PULL A SPARK PLUG AND TEST IT TO SEE IF THERE IS SPARK BY REMOVING AND SLOWLY TURNING IT OVER