Have a Z30 purchased from a BB employee used as a tester.
Unlocked, running on Bell network.
The OS has updated to 10.2.x, months ago; but there is no update to OS 10.3.
Is this a carrier issue, or is it a hardware issue?
OS updates on branded units is controlled by the carrier. Unlocked, unbranded devices purchased from Shop BlackBerry and Amazon are receiving updates independent of the carrier. In your case you may never see an upgrade, unless you do it yourself by unofficial methods.
I am confused by your statement regarding the past upgrade. Was that done by someone, or did you do it? Was it OTA or with Link, or some other method? If the device is getting updates independent of the carrier, that information might be helpful. For now you can remove the SIM and check for an update OTA on wifi. That is said to help. But if the device is under carrier control you won't get an update until the carrier releases it.
You probably would be better off doing an update by unofficial, unsupported method. Checkout Crackberry and learn about Sachesi and about autoloaders. You can do a non-destructive update using Sachesi. An autoloader will clear all data and settings. Either way, do a full backup for safety before you begin.
Koodo no longer carries Blackberry but is supposed to have 10.3 available sometime in February ( maybe). The OP is showing Koodo carrier and the Z30 is probably running the BB as an Android type. They also do not offer any type of BB data plan.
The web site states it "may" be available in February and BB Link wont see it until offered. As suggested by aiharkness, do it from blackberry directly but If I were in your position I would wait until it's officially available. why rush? 10.2 works