CA does not work
In some cases, carrier aggregation does not work.
Secondaries might not have been described in the cell
CA enters the added state if, while connected, a RRCConnectionReconfiguration message was sent from the tower, and the message contains secondaries. If no longer connected (RRC: IDLE), CA is removed. If it is not added while connected, the primary cell might not support CA, if CA stops working, take note of PCIs and Earfcn/DL changes, a certain PCI and Earfcn/DL may support it, another might not. The other PCI and Earfcn/DL may have been deemed more suitable, for example, because its signal strength is better than the cell that supports CA. For example, the phone may be moved to another tower that does not support CA.
Device might not support CA
Refer to https://www.globalcertificationforum.org/services/3gpp-certifications/all-3gpp-certifications.html. Category 4 phones are heavily implied to not support CA.
Phone might not be intended for use in your location
If the phone is not intended for use in your location, carrier aggregation may be unavailable.
Phone does not support the combination
Carrier aggregation combinations supported by the modem aren't enough to determine what can be used. The modem sends, receives, and processes coordinates of the signal, other components inside the phone do the rest. Necessary components in between, e.g. multiplexers, must support the combination, without potentially exceeding any design specification of another component.