[Sid Squid]

The comment above is correct - very few chains wear completely evenly, meaning that just about every chain that isn't brand new almost certainly will have a spot where the chain runs tighter.
This is not in doubt - the question is; can the chain be adjusted such that the tightest and slackest points are within acceptable limits? The difference can be so great that replacing the chain is necessary.
This is rare - but not unknown.

Kev: Go back to the shop, ask them to demonstrate the problem to you.

Chain adjustment changes as the suspension moves, when the front sprocket, the swingarm pivot and the rear sprocket are in line the chain will be at its tightest point, chain adjustment must take account of this, and if the figure for chain slack is given with the bike on its wheels a correction may be necessary if the adjustment is made with the weight of the bike removed from the suspension, that is; on a stand that supports from the pivot, not the swingarm.