Data gaps and discrepancies between what you think you exported and what you're paid for are a common problem, and there are a few usual causes. Communication problems with the smart meter are one: if the meter struggles to send its readings (poor mobile signal at the meter, or the meter sitting inside a metal box or shed that blocks the signal), ESB Networks may fill the gap with "deemed" estimated quantities rather than your real export — which can leave you short.
To protect yourself, fit your own monitoring so you have an independent record — for example a Shelly EM or Pro EM with CT clamps in the consumer unit, giving real-time import, export and generation figures, or reading export through the myenergi app if you have an Eddi. That way, when a bill shows fewer exported units than you actually sent, you have evidence to query it. A supplier can lose a handful of days of export and pay deemed (or nothing) for the gap, so a personal meter is worth having.
If you spot a shortfall, raise it with your supplier and, where it's a metering/data issue, ESB Networks — though be prepared for a frustrating back-and-forth, as the responsibility can bounce between supplier and network. The ESB Networks HDF (half-hourly) data download is useful for checking your own usage and export history, and recent changes mean it now tends to return fuller histories. Keep records, check your first few export bills closely, and don't assume the headline figure is right.