What is the spacing of the rear hub? Tandems are often 145mm (regardless of whether they are disc compatible or not), which is far wider than single bikes to give the rear wheel extra strength. The rear hub spacing might also be 135 or 140mm. On a typical road bike, it's 130mm and a MTB with a quick release it's 135mm.

Having determined the rear hub spacing, you need to determine whether the chainline of the crank and chainrings is matched to the rear hub spacing. With a 145mm rear hub, you would ideally need a 53 mm chainline (or at least this is what tandem-specific cranks are normally designed to have); typical road bike double cranks for single bikes have a 43.5mm chainline and road triples are 45mm; MTB triples are about 50mm. With some cranks, you can adjust the chainline slightly, but with many modern cranks with integrated axles, you can't change it.

Overall, I wouldn't be surprised that your chainline wasn't ideal, so access to all gears may never be possible. Any decent mechanic should know how to measure the hub dimensions and chainring positions to determine what kind of optimal performance is possible, regardless of whether the mechanic has ever touched a tandem before or not. If the mechanic is not capable of doing this, find a different shop.