Well, if the line is horizontal (flat) you might have given the Start- and End-Position the same values…?
(key-frame Start 0%, End 100% or vice versa)
Either select the key-frame in the Timeline-bar and in the Attributes under Key Properties change the Interpolation to Spline, or uncheck Auto Tangents and type in your desired values, or open the Timeline window, select Align to Spline, select the F-Curve-Symbol, select one of the key-frames, and adjust the handles to your liking, or if you’re still in Linear mode, right-click the key-frame and choose Spline…
Regards
Nolan