I probably agree with baffled and jams. A change probably is in order, but you didn't describe whether your weights are still progressing.
A decrease in DOMS may mean that your body has adapted to the workout, but that is not always the case. You cannot always gauge the effectiveness of your workouts by your soreness. Sometimes being sore just means that you are sore.
If your workouts are still progressing and you are able to perform more work, via an increase in weight, reps, or sets, then your workout is still effective. From your post, it is clear that you are still doing the same reps and sets but it is less clear if your weights are stagnating. If they are, change things up. If your weights are still going up, then you are still getting an effective workout.
The really bad thing with home gym machines, is that many of them have limited exercise options. Hopefully, yours is pretty versatile. If it is not versatile, then you can still change set/rep schemes.
If you really want soreness and you have a workout partner (you said we in your post so I assume you have a husband or significant other to workout with) then you can do some negative work. Have the partner help you with the concentric (positive) part of the lift and then do the negative under control. You can lift more weight in the negative so this may jumpstart your plateau in weights. The negative portion of the exercise is where the majority of the microdamage to the muscle fibers occur, so if you want DOMs, this is your baby. However, if you have been training under a year, I would not try this just yet. Your connective tissues and tendinous tissue between the muscle fibers may not be strong enough to handle this type of work.