If my Mantis has been sitting around several months without use, I choke it and it usually starts to sputter on about the 2nd or 3rd pull. After that initially sputter, I then take it off choke and pull again and it fires up.
I have had trouble starting mine on only two occasions and these were the causes.
Dirty Air Filter: My air filter was pretty dirty and I removed it, swirled it around in a coffee can with gasoline and reinstalled it.
Time for Tune Up: After some 3-4 years of use without any kind of trouble I started having trouble with it starting. I took it in to a Mantis-approved repair shop and they gave it a $35 tune up and it has worked fine ever since.
I suggest removing the spark plug and see if it is fouled. If it is fouled then your gasoline:oil mixture is likely too rich in oil. If the plug is just wet, dry out the ignition chamber with a hair dryer, or let it air dry and try it again.