When you have a late-running project there can be a temptation to bring more resource onto a project. However, the conventional wisdom is that bringing more people onto a late-running project tends to make it later. Why would that be, and what can be done?
In brief, because larger teams have more communication overheads, and because it uses up existing resource to bring a new recruit up to speed.
What can be done?
Firstly, address resource issues as early as possible so that new recruits have time to become productive. Secondly, hire experience - more senior staff require less time to familiarise themselves with the technologies used and become productive faster.