@faymansouri: This could be due to two reasons:
1) If you are burning the GP too deep with your heating instrument and as a result; by the time you condense the GP, you end up 2mm deeper than the orifice. IF that is the case, melt the GP 2mm above the orifice. That gives you a molten stub of melted GP that you should come back immediately with your larger plugger (size 10) to condense down. That creates a nailhead of GP that will spread around the orifice and seal the orifice and will not go deeper into the canal. This also helps protect your sealer from washout after you wash out the cavity.
2) Another reason for getting coronal voids might be because of having a larger taper coronal preparation than your Gutta Percha cone. If you’re using 04 taper preparation but are using lots of 08 taper orifice upper too deep in the canal, or you have a partially large coronal taper in the tooth, you may end with more room than a single GP cone can fill coronary. This is the case where you need to use additional auxiliary cones on the side of your main cone to fill-in gaps prior to burning the GP off with your heat instrument. You don’t need to laterally condense. Simply place next to it as deep as it goes and still try to melt off 2mm above the orifice for the same effect I mentioned above.
Trying to do the above two will definitely help reduce coronal voids in the cervical section of the root. Good luck! 🙂