A problem with all church calendars is that, by implication, they are trying to calculate when Passover ought to be. They think that Passover should begin on the first full moon after the vernal equinox, but it doesn't work quite that way.
All of these calendars rely on the convenient fact that the lunar and solar cycles fall into sync every nineteen years (modulo some fudging about leap years). The Christian and Jewish calendars, however, focus this synchronization on different parts of the year; Judaism concerns itself with the date of Rosh Hoshanah, while Christianity, of course, is concerned with Easter. Therefore the Jewish calendar uses a system of added months, plus some other adjustments, as explained here; but the Christian calendars use a cycle of full moon dates.
Besides this basic difference in emphasis, there are three other sources of inaccuracy. The first and more obvious is the Julian offset, now at thirteen days and growing by a day three out of every four centuries. But the full moon dates aren't accurate either. Some degree of error is inevitable, but as it happens the old paschalion just gives flat-out wrong dates at times. Finally, as it happens the Gregorian calendar is a little off, so that sometimes the equinox actually falls on the 20th instead of the 21st of March.
In an attempt to get all of Christendom on the same calendar, a group met in Aleppo, Syria in 1997 and came up with a new formula. In spite of much Orthodox participation in the conference, this proposal sank like a stone.