Tried to check in and select seats for my flight tomorrow at the 24 hour mark. Told:

"We have found your reservation, however we are unable to check you in online. Please proceed to the airport ticket counter and an agent will be happy to assist you. We apologize for the inconvenience."

Why did I get this error message? I am flying from SLC to DCA via DEN and MKE. The segment from MKE to DCA is on Midwest, so I suspect that that is where the problems are. But, if so, why won't the system at least let me select seats for the first two segments, particularly since Frontier and Midwest are both owned by Republic.

This is not giving me warm fuzzies for Frontier/Republic. I typically fly to Washington several times a year and usually go on a Delta nonstop to DCA or BWI. This time nonstop was not an option because the fare cost exceeded (by a mile) the cost differential between nonstop and connecting that my company travel regs permit. So, anyway, I was not happy about flying on Frontier anyway and this issue gives me no reason to change my mind about using Delta as my first option.

According to Seat Counter the flight from DEN to MKE is 0 accross the board. So I am worried that when I check in tomorrow morning at SLC, I will be told, "sorry, no seat for you." Does Frontier have interline agreements with anyone?

Right now Frontier and Midwest have different computer systems, so you cannot check-in online if one of those segments is operated by the other carrier. I think they are planning on merging the computer systems later this year. Try checking in for the Midwest flight at www.midwestairlines.com

Other reasons for failed online check-in are that you are on the TSA "selectee" list, or you're traveling with an infant, or the only available seats are emergency exit rows, or some information is missing from your reservation, like birth date (now required for TSA).

I am not in a mood to cut Frontier any slack. Even if I cannot select seats on the Frontier system for the Midwest flight, since that is the last of my three flights tomorrow why could Frontier not at least let me check-in and select seats for the first two flights?

For the record, I am not on the selectee list (I believe there is only one other person in the country with the same first and last name combination as me). I am not traveling with an infant, seat map shows lots of other seats unselected besides the emergency exit row (at least right now -- when I get to the airport tomorrow, who knows?), and I supplied my birth date.

You should be fine when you checkin at the airport with all of the other economy tickets. I wouldn't be more worried about not having a seat than with any other airline, but you may not get a good seat depending on the number of people with economy tickets that were able to check in online.

When you check-in on any airline it is always for the full itenerary (for the day of travel). You cannot be checked in for only 2 of the 3 flights.

It sucks right now but they are working on it. Merging two different IT systems is an enormous task but they want to be done by the end of the year. At least you might be able to earn skymiles?

Had same problem in December when flying on Midwest ticket with Frontier equipment. I could view my reservation on Midwest's site, but not check-in. Could not even use a kiosk at DEN ticket counter. The flip-side, as noted above, is that on both legs I ended up in the first 6 rows of the plane.

I booked on Frontier's site and did not have that option. I found out the next day by reading posts on FT that Skymiles can be chosen if booking via Midwest. Since Midwest does not have operations in SLC, I didn't even think about looking at their site. Oh well.