This condition is common with some cows. In fact, I have seen a cow that has to be helped to expel the placenta at every delivery. To help the animals under this condition, there are some steps that could be taken:
First, the animal can be given a drug that will aid contraction which will help expulsion of the placenta. E.g. oxytocin can be used for this. Meanwhile this must be used under the supervision of your consultant.
Second, the placenta can be pulled out manually whether as the only approach or after administering oxytocin. After pulling out the placenta, the animal should be placed on an appropriate antibiotic for the recommended number of days by your consultant.