yo, you've probably figured out by now that you need to switch the SYNC SOURCE of your sp-16 in order to successfully start the sequencer.
samples set to LOOPS will of course play when manually triggered but that's because they are freely looping opposed to looping ON the sequencer.
if you couldn't find this info in the manual (cf. below) it's not because you missed it. You couldn't find i because it is not located in the official TORAIZ SP-16 OPERATING INSTRUCTIONS. the info is instead found in an equally important though rather luridly titled document named: "FIRMWARE VERSION.1.60 OPERATING INSTRUCTIONS ADDENDUM".
i'm sure there's a good reason why AlphaTheta seems committed to addendendizing (probably not a word) their manuals opposed to releasing a single updated document, but there you go.