The reason TC can be "too busy" is because the route involves clearing the ac down to almost min stack E of logan amonst other factors such as shall not delay other traffic etc, pretty much unless it is the only thing running in to Lam you cant approve it, likewise by the letter of the law if you want to issue a second arrival the Prnav procedure the first a/c must be with LL apc and be confirmed off, so it really only needs a couple of arrivals for it to be too busy to use this procedure, even though it is relatively quiet. Also AC should not be clearing the ac for these arrivals but be transfering them in plenty of time for TC to do this i have experienced a couple of crews who were under the impression they were cleared for this approach on first contact with TC.