Your comment, understandable from anyone not familiar with these engines, illustrates well why so many folks seem to run into problems.but my other cars that do use 'belts have fairly obvious timing marks on the respective pulleys which would enable an initial inspection and diagnosis if this is indeed the problem. Given the criticality of a timing belt change I would HOPE that whoever did the work would at least get the marks aligned.
There are no timing marks to align; the cam sprocket isn't keyed, and can be set to any position on the camshaft, so the valve timing is infinitely variable. This does mean you can set the timing precisely to compensate for any small variation in component tolerances, you're not constrained to aligning with the nearest tooth, but it makes the belt changing procedure both non-standard and more involved.
So it makes no sense at all on these engines to talk about the timing being 'one tooth out', since it can be set anywhere. Indeed, if anyone does suggest the timing is a tooth out, it's a sure sign they don't understand the way this engine should be timed.
Even checking requires the correct timing tools, and is practically as much work as changing the belt in the first place, so this isn't something that can be quickly verified.
The Euro5 and later versions are interference engines, so get it substantially wrong and you'll cause some serious damage.