(Some) add-on upgrades not working (Concrete5 5.5.1)

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I can update C5 and all add-ons just fine. And overall my site is not displaying any problems. But I have an issue upgrading two add-ons, and I don't know if the problems are related (we have C5 5.5.1):

(1) SEO Manager Add-On. I have 2.5 installed and my C5 sees no update, though 2.5.1 is out. I have contacted the add-on developer but he had no clue as to why this is happening.

(2) Authorize.net Payment Form Add-On. I have 1.0 installed and C5 DOES see the 1.01 upgrade. When I click "Upgrade and Install" it goes through the process and posts "The package has been updated successfully." But it hasn't.

Any suggestion? Please help.

bcarvalho