A recent study from Realtors in June 2019 shows that 76% of real estate sales contracts close on time.
There are 4 reasons they don't close on time, which are:
1) Financing issues;
2) Appraisal;
3) Home inspection or environmental issues;
4) Title and deed issues.
Financing issues usually arise from the buyer purchasing a big ticket item during the escrow period, which changes their debt to income ratio or affect their credit rating.
Appraisal issues involve the home's negotiated price versus what an appraiser says it's worth. An important part of my job is to make sure you aren't paying too much, so the property does appraise.
Home inspection and environmental issues can occur from a home owner not agreeing to make repairs, or the discovery of unknown issues on the property. For example, one of my clients selling their home in the DC area had his closing delayed after a buried 1940's gas tank was found in the yard.
Title and deed issues can be fixed by the closing agent, but will require signatures from the home owners, which can cause delays.
While a delay in closing isn't the end of the world, I understand that your plans to pack up and move are centered around a reliable date. It's a big part of my job, besides helping you locate the right home, to make sure that your closing occurs on time.
Source: Realtors Convidence Survey: June 2019, National Association of Realtors