Sleep trained baby why short naps?

My baby is 6 months and has been sleep trained recently as in sleeps with no aid at 7 and wakes up once for feed till morning and sleeps overall 10 to 12 hours, but his naps during the day are all very short 30 max 40 mins, as I understand 30 mins is one sleep cycle and he has to go back yo sleep even if he wakes up, but he just has wide open eyes after 30 mins and I have to move him from crib, what am I doing wrong?