US Airways Dividend Miles (Pre-FlightFund Merger) - Check-In Kiosks- check in closed?

OK yesterday coming out of NYC to LGA for the 6pm Shuttle flight, I got a cab at 4:55 and made it to LGA by 5:30. I went up to the Check in kiosk and treid to retrieve my boarding pass for my 6pm reservation. went through the normal questions, and just before the machine is due to print out my broarding pass-- I get a message that Kiosk can not give me a boarding pass due to the check in time and I am to go over to see a agent. Well-- guess what-- 2 agents are working the counter, 15 people are on line, no elite check in for Shuttle flights. So I wait, wait, finally 5:50 I get to an agent--luckly boarding pass is in my hands in a few seconds. Run to secuity--Laptop already out of the bag-- Everything goes through no problem, but they want to x-ray my shoes
Shoes off-- through x-ray-- shoes get cleared. Down to the gate-- just about to close door and get on the 6pm flight. Now for futere reference, when does the Kiosk close for flights-- it has to be at least 30 minutes of less.

I have had the same problem with the kiosks at LGA flying the shuttle operating between LGA and BOS. Recently I arrived at the shuttle area kiosk at approximately 0635 hours, and got the same message that I was too late to check in, and checked in ok at the adjacent counter. However, in contrast, at BOS I am nearly certain that I have checked in at a kiosk at the shuttle at like t-15 minutes to departure without issue.

One thing that airlines do close to departure is restrict check-in and other transactions so that only the gate agents operating the flight can check in passengers. This is usually done between 15 and 5 minutes to departure, and each airline has a technical jargon like name for this practice. For example, at CO they call it "restricting" the flight. I.e. a ticket counter agent might call the gate and say "could you unrestrict 285 to CLE for a minute," etc. I don't know what the cool tech term used at US is?

However, this would not explain why the counter, in these shuttle LGA anecdotes, is still able to check in the pax. One would think that if a flight is restricted only the gate could check pax in. However, I do believe that the shuttle uses something called Shuttle Sabre which is Sabre with some nuances specifically for the shuttle.

All that being said, I have no idea why they would be closing flights to the kiosks at t-30 t-20, etc? And why only at LGA apparently? The ticket counter agent had no idea why this happened. And I lost a mere 50 miles in the process as well, even though I was at the ATO nearly with in the 20 minute guarantee.