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Start by testing the opener, you should be able to disconnect the door and open it manually. If the door tends to stick in the same spot, when manually opening then you know it is not the opener. If the door rolls smoothly, then the opener is either missing a tooth in the gear system or the door is too heavy for the opener.
If the door is sticking, when manually opening then I would inspect all tracks for bent areas, inspect all rollers and inspect all springs/ pulleys/ wires are operating correctly and not getting caught or rubbing severely on other parts.
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