Hi Pegasus,
Sue's comment about garden soil being not the best medium for pots is good, and possibly a factor in the problem. The problem is likely a nutritional one. Do you know if your 19-19-19 fertilizer contains trace elements?
Also, what do you know about the water that the pots get? Is it rain water or tap water or what? Doing a soil test of some sort of the soil in the pots could help, even if it was just a pH test, using an indicator paper. Verbenas need the pH to be in the range 5.5 to 7.0. A pH outside that range can affect nutrient availability, regardless of what fertilizer you supplied.
ZM