The most frequent hold-up is an incomplete pickup address — a postcode without the specific house, flat or building name in a densely built-up area, or a large office or hotel without a note on which entrance to use. Adding those details at booking rather than leaving them for the driver to work out on the day means less back-and-forth by phone right when you are trying to leave. A similar issue comes up with terminal numbers on multi-terminal airports: confirming your terminal against your actual airline booking, rather than assuming from memory, avoids a driver being positioned at the wrong one.
The other common gap is luggage that changes between booking and travel — an extra case bought while away, or a different group size on the return leg than the outbound. Updating us as soon as that changes, rather than assuming the original vehicle will still cope, means the right car is arranged rather than a squeeze at the kerb. None of these are complicated to fix; they are just easiest to avoid altogether by giving slightly more detail than feels necessary when you first book.