VIDEOJS: playing a pre-roll add before each playlist item

I'm using the videojs-playlist plugin along with Google's videojs-ima plugin. Everything works swimmingly except I am only getting a preload ad before the first video. I want one before each video in the playlist. Basic setup is boilerplate, but for reference:

this.player = videojs('currentvideo', { autoplay : autoplay, fluid : true });
this.player.playlist(this.playlist);
this.player.playlist.autoadvance(5);

const skippable_linear = {google's test ad};
const options = {
    id: 'currentvideo',
    adTagUrl: skippable_linear,
    debug : true
};

this.player.ima(
    options
);
this.player.ima.requestAds();

I have tried various ways of manually calling ads from inside an 'ended' event handler, such as calling requestAds again:

const _this = this;
this.player.on( 'ended', function(){
    /* some other stuff */
    _this.player.ima.requestAds();
});

This does play an ad where I want it, but

1. this breaks autoplay (next video doesn't start playing when the ad is finished), and
2. this puts the player into "ad display" mode (scrubber is unavailable, etc).

Is there a simple way to just say, "play an ad now" programmatically? I've tried, without joy, to use all of the seemingly applicable methods exposed by both the ima plugin and the contrib-ads plugin it relies on. I'll admit here that this is the first time I've ever had to deal with videos that run ads, so I'm kind of a noob.

via axlotl