Some connections it might not be selling, maybe because of the layover though.

For example, on Sat 5/16, there are
SFO-IAH (8:55am-2:50pm), Flight 1106

IAH-PHL (6:50pm-11:10pm) Flight 242


But Frontier's site isn't selling SFO-IAH-PHL.

However, the layover is 4 hours so separate tickets have to be purchased. If the layover < 3hrs, it probably would be sold.

In comparison, JetBlue does sell PHL-BOS-DTW, two flight itineries with an over 4 hour layover in BOS, and one itinery with a layover of 51 minutes. JetBlue isn't selling PHL-BOS-RDU/PIT even though I recall PHL-BOS-RDU even having flight times that could be sold as a connection.