This tracing confirms that atrial lead is not capturing the atrium, but it appear to be firing at a rate less than 75 bpm, which means it senses "something" that resets the PM. 

So, the atrial lead is likely dislodged and when this tracing was taking is just passed the TV and hence was partially sensing the ventricle (to reset the timer) but does not capture the ventricle. 

Illustration below...

On the top of the EKG tracing the machine will mark the location of the PM spikes (black short vertical lines at the top of the EKG). At the early part of the tracing, you can see that the PM fires regularly at 75 bpm (except for the first spike), however, after that it will fire at a slower rate (the distance between PM spikes gets wider than the programmed rate). This indicates that the PM was sensing something and hence adjusts its pacing cycle accordingly. 

The spikes seem to be occurring at a consistent distance from the preceding QRS complex (black arrows) which is the same timing cycle of the PM (red arrows - about 800 milliseconds). This suggests that the PM is probably using the QRS complexes to set its timing cycle (in most part), and hence it is probably sensing these QRS complexes (likely intermittently due to lead instability, which explains the lack of consistency – note it failed to sense some of the QRS complexes like the second beat on the tracing (*)), so it should be close to the RV (i.e., the TV area, likely just passed the TV).  

The EKG below was taken while the PM was programmed in AAI mode LR of 60 bpm with the patient doing some maneuvers to assess lead stability. What is happening now?