- Price — Since the spot price can rise above your bid price anytime. If the Spot price is greater than your maximum price, the instance will be unavailable.
- Capacity — If there are not enough unused EC2 instances to meet the demand for Spot Instances, Amazon EC2 either interrupts Spot Instances or new Spot instances are unavailable.
- Constraints — It might sometimes happen that a spot instance is unavailable in one AZ, however, it is available in another AZ of the same region. So, if your request includes constraints like to launch the instances in a particular AZ and the capacity is not available in that AZ, then your request would not be fulfilled.
For more information, please refer: