A worn key is a distinct possibility. Coupled with worn tumbler pins in the most frequently used locks (ignition and driver door) it can start making for a problem. Initially wiggling a key may help. A bent key doesn't help if it binds in the keyway or tumbler pins don't ride properly.

If trying a spare key works well then a locksmith should be able to trace cut another key if they can find an aftermarket blank. Note that there are two types of keys that came with the car, the master key that fits all locks and the valet key that only does the door and ignition. The difference is only the blank keyway slots, not the cut. When you try a spare key, if it doesn't work in the trunk then that's why. Also, Volvo sells both types of key blanks, but locksmiths can usually only get the master blank unless they go through Volvo.

If trying an alternate key isn't an improvement then consider having a new key properly cut as you did last time. Some locksmiths may be better at this than others.

Start off by talking to your local Volvo Dealer Service to see if thay can make the key. First off, the genuine Volvo keys are stainless steel quality that don't wear as quickly and aren't easily bent, unlike most of the generic blanks available. The key code number was in the Owner Manual. It was not stamped on the keys. If you no longer have the manual then the dealer can get the key code from Volvo using the VIN (presumably wanting proof of ownership). Your dealer has (at least used to have) a key punch and die kit that punches a proper new key rather than cut grinding. For a well used key cutter at the dealer, the dies as they got worn would sometimes cut less than a perfect key that may be difficut to use in some or all locks. Burnishing the key heavily with a wire wheel, multiple times if necessary, would usually get the key working properly.

Failing the dealer being able to help, most locksmiths, like the one you used before, should be able to cut a reasonable facsimile starting with the VIN and waiting for a response from Volvo. Sometimes they also need to order the blank from Volvo if they can't find an aftermarket blank, also meaning you get a good quality key. You can tell a genuine key as it should be the only one with the trademarked Volvo name on it. Ideally, have the car at the locksmith shop or use a mobile locksmith so they can ensure the key works smoothly and can lubricate the locks and burnish the keys as needed.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now