You might need to determine if your watch been adjusted to run on the higher voltage (1.55v) of the modern silver oxide cells, opposed to the 1.35v of the older mercury cells. Some Accutrons run well without the adjustment (re-phasing), others don't keep time without it. If yours hasn't been adjusted, Borel and CasKer both offer a specialty cell called Accucell-1, which is a smaller cell fitted in a properly sized housing that uses a semiconductor to reduce the current to the original 1.35v. It's a reasonably priced option to having the watch rephased.
Originally, the 214 should have used a 387 Mercury cell, the 218 used a 343. Typically the modern 1.55v cells like the 344 cause the older Accutrons to run fast, or not at all. Sometimes they will time well on the bench and then gain on the wrist. Slow running is not normally a problem due to the voltage of the silver oxide cells.
If your watch was keeping time before the battery change, and is consistently slow after, it may be because poor contact is effectively reducing the voltage from the battery. Check to be sure that there is no corrosion in the battery well, and also that the plastic spacer isn't interfering with the battery being seated properly. Seat the battery firmly, but don't force it, to ensure good contact with the coil.
If your watch wasn't running, and hadn't run for a long time, then it likely needs to be serviced and re-phased.