Re Shorter distance issue

Hi Garmin Support team

I have been using Garmin R10 for about 3 weeks now, it seems all the shots I made was gauged about 15-20 meter short, the longer club I use, the bigger variation it has, I am playing with it outdoor at my back yard to the net. the distance hasn`t improved after upgrading to 3.70. I haven`t try 3.8 yet but guess it is not the issue.  

can anyone help?

Thanks