After upgrade the SDK to 6.2.1, My watchface couldn't be downloaded by some watches

Sometimes I wasn't able to install my own app from the store. I knew it works, because I could side load it, and I had free app slots, but it turned out that there wasn't enough free memory. So even if someone sees your app in the Connect IQ and sees he has at least 1 free slot, it's not enough. Usually deleting some other apps freed enough memory to enable installation.

Also it's not clear from your report if people who reported the problem do see your app listed in Connect IQ, and nothing happens when they try to install it (in this case it would help to get more details on what exactly that "nothing" is) or they don't even see the app listed.

Sometimes I wasn't able to install my own app from the store. I knew it works, because I could side load it, and I had free app slots, but it turned out that there wasn't enough free memory. So even if someone sees your app in the Connect IQ and sees he has at least 1 free slot, it's not enough. Usually deleting some other apps freed enough memory to enable installation.

Also it's not clear from your report if people who reported the problem do see your app listed in Connect IQ, and nothing happens when they try to install it (in this case it would help to get more details on what exactly that "nothing" is) or they don't even see the app listed.

After the FW update, reconnect the watch with Garmin express oo install the apps.  Or make sure the watch has synced with GCM.  Those are how the app store will know the FW has been updated.

Sorry, I didn't make me clear. They cannot install the watchface because they were asked to upgrade their watch firmware by "Garmin Express". Even they'd done that, the error still exist. That issue is only reported after I upgrade the SDK to 6.2.1.

Another thing to note about total CIQ Memory (it's not used for sideloads).  Lets say the total available space is 15mb, and 14,5mb is currently used.  One of the apps, that has a 1mb prg file gets an update.  Doing that update will fail (you get a message with Garmin Express), because there must be enough space for both the current version and update at the same time when installing the update.  So you always want to have enough free space to allow for an update of your largest installed prg file.

For new Garmin users, it's also not uncommon that someone installs an app, it shows as installed, but they don't know where to find it on the watch.  They install a device-app for example, but don't know it will be at the end of the list of activities when the start button is pressed from a watch face.  Or they don't know how to use a data field in a native activity.