From your 2021 MCME Interactive ECB session:
If the placenta is IN (has not been delivered) DO NOT perform external uterine massage (a.k.a. fundal massage). This can damage the placenta and cause further bleeding, as portions of the placenta may be retained and inhibit the uterus from contracting down.
Instead, complete your oxytocin stimulating techniques (skin-to-skin, nipple stimulation/latching) and if this is unsuccessful, external bimanual uterine compression and transport to hospital.