Why is my chest press the only exercise where I’m not getting stronger?

I should preface this by saying that my strength workouts are secondary right now; I’m training for a short distance triathlon so biking and running are my primary workouts along with some swimming, and I add a session or two of weight machines at the gym per week. I’ve lost a bunch of weight in the last year so I know maintaining/adding muscle is important in general as well as supporting my other athletic goals. 46F, 5’7, 160 lbs (was 245 a year ago!)

Keeping my workouts simple, based on a beginner routine I found online. After a brief treadmill warmup, I do lateral pulldown, seated leg press, chest press, and abdominal crunch. For each of these, I do a warm up set of 12 with lighter weights, then aim for two sets of 12 with my target weight. If I’m successful at a given weight for a time or two, I’ll add on the next time; otherwise I stay at the same weight until I can complete a session. If I don’t finish the first set, I drop back down to the previous weight for the second set. I’ve started adding assisted pull-ups at the end just because I think it would be cool to be able to do a proper pull-up eventually; I do three sets of 5 with decreasing weights of assist for each set.

For most of the exercises, this is working and I am progressing to heavier weights on a regular basis. I just can’t get anywhere with the chest press! The machine has two stacks in increments of 10 pounds, plus a 5 pound add-on weight on each side. I can lift 10 per side for a total of 20, but I cannot for the life of me get through two sets of 12 with 15 per side/30 total. Should I be doing shorter sets? Maybe three sets of 8 instead of 2 sets of 12…I can usually do the first set of 12 and then anywhere from 6-9 of the second set, so sets of 8 might be manageable. It definitely feels like this is a small amount of weight to be struggling with! Is it normal for this exercise in particular to be weak?