What should I do when 2 different gps units cannot locate a geocache?

Have used a Garmin Rino and Geocaching.com app on iphone and both keep getting close to cache. When each would update, cache would be in another area. Even with just searching area, no cache was found, but my problem is mainly with the hit and miss of gps units. Any ideas, hints or tips would be greatly appreciated. Thank you

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