I replied to you via email already, but I'll post it here so that everybody knows too.
Version numbers are not decimal numbers.
By the PHP standardized convention, version 4.2 is lower than 4.18.
Version numbers are independent integers, that just happen to be connected by dots.
True, in version numbers:
4.2 < 4.18
4.20 > 4.2
4.2.1 > 4.2
4.2.3 < 4.20
Get it? The version_compare is telling the system that 4.2 is not an upgrade to 4.18, so they're not going to get notified and they won't upgrade.
Change your version number to be higher than 4.18. Then they'll get an update.
BTW, You already messed up your version numbering something royal with that 4.171 version. To get those sites to update, you'll have to go higher than that, like with version 4.172 or 4.180 or something.
Might as well just switch to version 5.0 and then start using version numbers correctly to begin with.