outlookService.Sync(Outlook) function is not working

Hello all,

 
I am using 2015 on-premises. I had configured outlook to CRM properly but its not sync properly,sometimes its taking too much time . Its not sync in every 15 min,sometime its syns in 30 min, sometime 15,20....50min as well.

for resolving this issue i am made a window service through that i am sync outlook to CRM.
But in code i am using ---- outlookService.Sync(Outlook) function.
its not triggering sync in outlook,please suggest me where i am going wrong and what i have to do for that.