Could it be anything else? They are not yellowing at the tip and I see no evidence of a bug, (yet)!
If it were a watering issue there would be signs of browning on the leaf margins. If it were a fungal problem there would be darkening on portions of the leaf and stems. Scale on the other hand is a sucking insect and there are 2 types. Hard and soft scale. Both cause yellowing. What I believe you have now is soft scale but if not treated will also become a hard scale issue. I magnified the last picture. It shows very tiny white spots on many of the leaves. Look closely as these things are VERY tiny. Get a magnifying glass and look at leaves. When you see a white spot rub your finger over it and if it smears it is soft scale.
I posted earlier that it isn't a watering issue. But how about the plants being root bound? Have you checked lately? You may be watering correctly but the plants may not be able to uptake the water because of compacted roots. This will weaken the plant and invite unwanted insects such as scale. I would check this out. Use a neem spray for the insects.
I just thought it was worth mentioning as a reason for this. If repotted last year they shouldn't be root bound if planted into a substantially larger container.
