Originally Posted by emcampbe

Is this booked already, or not?

If you want to try and force a shorter connection, try using multi-city and searching for XXX-FRA as one segment, FRA-PRG the next (or MUC, or whatever). You'll get more options that way, or use Google Flights, and use the link to book with UA (you can filter for airline, or connecting point).

Put simply, UA's search engine is awful. While it's possible shorter layovers involve a flight that is sold out, more likely that UAs site is simply missing one (or several) normal/reasonable connections that shouldn't be a problem to book. Living in a spoke, I always check with multi-city or Google flights before booking on UAs site since this is frequently an issue.