Yup, sounds like chain slippage.
Setting up an SS for the first time is nowhere near as easy as it looks! We've all been there though!
The trouble is, "traditional" mountain bike chainstay length is 16.75" which is about a mm or two too long for most usable SS ratios. Even with a tensioner the chain is just to long to be "tidied up", as you will need to remove two links to make it (a whole inch) shorter. You can use a half link with KMC chains, or stray a cog or two away from the 2:1 ratio, or use a slightly stretched chain.
I use a 33;17 ratio on my Soulcraft with the dropouts slightly filed by a mm or so to accomaodate a fresh chain with no tensioner. This gives me a mm of adjustment too as the chain stretches.
You might find this useful. http://eehouse.org/fixin/formfmu.php
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