Does your baby wake shortly (30-40 minutes) after falling asleep at bedtime? This is known as a “False Start”. It affected one of my twins at around 7 months old - she would wake 30 minutes after going to sleep. We actually found it was down to under-tiredness and gradually began to increase the wake window from nap 2 to bedtime.
If your baby wakes shortly after going to sleep, consider the following:
Over/under tired: Was your baby sufficiently tired going to bed or were they under/over tired? How was their form before bed? Watch their sleepy cues and consider adjusting the second nap and wake window, as appropriate.
How did your baby fall asleep? Was your baby very drowsy (nearly asleep) going into their cot? Try having your baby a little more awake (but calm) going into their sleep space to ensure they are using their self-settling skill to fall asleep. This will also allow them to be fully aware of how and where they fell asleep.
Was your baby transferred to their cot once asleep? If a baby falls asleep in one location and they are then transferred to their cot once asleep, your baby will realise they are no longer in the same place they fell asleep.
Bedtime routine: Do a calm, consistent bedtime routine to ensure your baby is relaxed and not over stimulated before going to sleep. Have the lights dim to enhance the production of melatonin, the sleepy hormone.