Does the OP-1 slow down after a while?

I’m a fairly new OP_1 user , maybe someone out there can suggest a solution to a problem I have encountered?
I have added some samples to my OP-1 and very carefully mad sure they are at middle C but when the OP-1 takes them and converts them to a patch the note shifts. The first one (a Hang Drum) shifted up to an A, I corrected this by shifting the sample down by the same amount. So I then tried a 12 string guitar sample using the same method but it turned into a G# and took quite a bit of trial and error before I managed to get it about right.

My question is this. How can I get the sample to the correct pitch for the OP-1 to convert or how can I edit the OP-1 patch to correct it?

Any suggestions gratefully received.
Thanks.