Implementations tend to fail on a schedule. The first months go well: selection, configuration, demonstrations. Somewhere around the fourth month the pace collapses and the project acquires the word “phase”. The causes are consistent, and none of them are technical.
Nobody owns the data
Migration is presented as a task. It is actually a series of decisions: which customer record is correct, what the opening balances should be, which of four product codes survives. Those are business decisions, and if no one with authority is assigned to make them, they queue.
Name a person per data domain, with the authority to decide and the time to do it. This single step removes more delay than any amount of additional technical resource.
Two systems, same people
The other reliable cause is asking the same staff to run the old process and build the new one simultaneously, on top of their normal workload. Something gives, and it is always the project, because the old system is what pays this month’s invoices.
Either backfill the operational work or reduce it explicitly for the period. Assuming the capacity is free is how a four-month plan becomes a fourteen-month one.
A date nobody would defend
Ask the people doing the work whether the go-live date is achievable. If the honest answer is no and the date has not moved, the project is already managing appearances rather than progress.
Better to set a scope you can deliver on a date you believe, and go live with less. A narrow system that works builds the confidence to extend it. A broad one that goes live half-ready teaches the organisation that the new system is unreliable, and that impression is expensive to reverse.
Most of this is ordinary management attention, applied early enough to matter.