Did you buy the bike new? Did you change the gearing? If the chain and cogs are as they came from the factory, it would be extremely unlikely that the chain is too long. The first illustration on this page shows how to determine whether a chain is too long.
Is that what you're experiencing? (Note that the mechanic shifts the bike into these gear combinations ONLY to test chain length, and they should not be used in general, because of the stress on your chain; hopefully your LBS explained that to you!
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If not, why do you think your chain is too long? If by "not catching" you mean the chain is rattling in several gears rather than positively engaging a cog, then you likely need a cable adjustment - as Maryellen said, new cables do stretch and need to be tightened several times over the first few hundred miles. Hopefully your LBS offers a break-in service that includes adjusting all your cables. If you do it yourself or ask a friend to do it, don't neglect your brake cables. If it's rattling only in a lowest or highest gear, or coming off the cog entirely, then the limit screws may not be set correctly.
Speed comes from what you put behind you. - Judi Ketteler