If you haven't used Reiki in awhile
 

I got attuned to Reiki III like 10-15 years ago.
I used it to help a dog whose buddy had passed away.
It stopped whining after I sent Reiki for like 10 minutes.

I use it on my aches and pains at times.

But it's not been consistent.

Does more experience and usage of Reiki make it more effective and powerful?

My mom is in a lot of pains from Fibromyalgia and arthritis. I try to send it when I can but it doesn't do anything that I know of.

My Reiki teacher used to say that if you do not use Reiki it is as if you have a Porsche in your garage but you don't drive it. You still have it though. If we take this analogy further, I think a car that hasn't been used for years won't start ;)
I have been attuned to Reiki III and I hardly ever use it. I noticed that when I try to do Reiki now, I hardly feel anything. After Reiki I the effects I felt were great. I felt energy rushing through my body, but nowadays it's gone. I got the feeling that it might start running again if I started practicing regularly.

Start out with some self alignment/self healing to get back into the practice. You never lose the attunement, so it is always available to you. Remember you are only channeling Reiki; it doesn't come from you. Just open yourself to it an let it flow through.

Peace