Is the starter just stalling, or is the rubber slipping on the prop ?
Also are the engines the same brand as you have been running in before - or different ?
There is a huge discrepancy in the "tightness" of new engines from make to make.

I'm guessing your pre heating the engines before starting ?
Is it possible that whatever you are using to heat the engines is not working properly anymore.......
Not properly heated a new engine will be a struggle no matter what starting system your using.