Reporting
Anne, it's not clear if you

Anne, it's not clear if you dad's other dahlias grew but did not produce flowers, so we will assume that's the case.

Nonetheless, there is no easy answer. It may be that the nonbloomers got too much nitrogen (first number of the three on a fertilizer package should be low) and/or not enough light. Basic top soil is adequate.

That said, at least one source recommends giving the plant/s fish emulsion.

Another: Grow in morning sun, and be sure they get 6 hours of sun per day.

That's all we can say about this. The color factor is a puzzle. Why not ask the neighbor if he/she ever had this problem.

Best wishes!