yeah i realize the same also. when i first used muscle testing a while ago, i had an incredible session that i still cannot reproduce even now.
I had two friends over who then knew nothing at all of muscle testing... i had one of them pick up two random cards from a deck of cards, and id muscle test my other friend (both of us had no idea what the cards were) to try and figure em out. It was incredibly accurate.
i tried again 4months afterwards and then it didn't work anymore, although it might simply be because i then was on a lower consciousness level.
...Ok I just did a bit of finger dowsing using this example, here i share it... it might be useful for you and others to get an idea what can determine the limits of ones ability to dowse.
Using the scale of consciousness put forth by David Hawkins in the book Power versus Force, one needs a consciousness level over 360 to be able to determine two unknown cards that somebody is holding.
The first time around i tried this I was at 460 consciousness level, hence why it worked.
the second time, i was at 270, hence why it didn't work.
so, going with this idea, the higher your consciousness level is, the greater your reach in the unknown will be...