Key reasons for not getting an IPO allotment are:
IPO oversubscription and allotment is done through a lottery
Application rejected due to mismatch/incomplete information
The issue price is higher than the bid price
IPO Oversubscription
Based on demand and supply, it is possible an IPO oversubscribed (received more bids than the available shares for the public).
Here are the two cases of oversubscription to allocate shares.Application Rejected