Your new batteries are likely only at 30% charge. They need to be charged and balanced before you can reliably place them in series.
First, you charge each of the 12V to 14.4V and hold for two hours.
Then you charge them in parallel to 14.4V and hold them for two hours.
Then you place them in series for 24V.
If you want a slow charge that maximizes cycle life: 27.6V boost, 120 minute boost duration
If you want a fast charge to minimize charge time: 28.8V boost, 30 minutes boost duration.
Float: 27V
temperature compensation should be 0 or disabled.
equalization should be same as boost and/or disabled and/or set to 0 time.