No start at times

I have a 2008 forester with 55,000 miles on it and three times it has not started in the morning. It turns over fine...just will not start. I test the battery and have appx 12.75 volts and after I get it running I have appx 13.50 volts. I assume the altanator is working!
The method that I use to start my car when this happens is to run jumper cables to my truck and start it up. This works even though as I said my car turns over just fine...but will not start. The battery connections are perfect as are gas/air filters. Sooooo
I brought it to the dealer (2nd time) and he says I need new spark plugs. I assumed that a plug change was not needed untill 100,000 miles as all my other cars...no big deal, I'll change them buy I seriously DOUBT this will solve my problem. No check engine light ever came on therefor I never checked for codes...but I intend to now.
Any suggestions! Thanks

Why did you asume that your spark plugs need to be changed every 100000?

I have regular plugs on my 2010 that require 30000 miles change intervals.

What does your owner's manual say?

Sam

My F-150 and 2 toyotas have plug change intervals at 100,000 miles. My subaru runs fine...smooth with full power so I never considered that to be the problem. I still don't. I will change them because as you stated that is required at 30,00 miles.
Thanks!