They look like immature scale and they can be persistent. On the main stems, do you see round reddish-brown or black hard circles?
Gene has a good suggestion! Another option is to try a spray bottle with half water, half alcohol and a drop or two of dish soap. Spray it all over the plant, every leaf (top and underneath), every nook and cranny. Repeat in 4 days, then in another 4-7 days. The water-alcohol method is probably a little slower. Depends on your patience/preference.
If you can, get the plants that have the insects away from the others (don't let them touch) because that will slow down the spread of these buggers.
