The rhythm in the first strip is paced atrial rhythm that is irregular. In this case the PR interval is increasing like that seen in Mobitz I A/V block. In this case though, there is no A/V block, but as the PR prolongs it eventually extends past the atrial refractory period of the pacemaker (400ms) and the QRS is sensed by the pacer and hence inhibits the pacemaker. The pacemaker is working well.

The second strip shows a regular rhythm where the PR interval is short enough so that the QRS stays within the atrial refractory period and does not inhibit the pacemaker.