Do you do chest 2x week? If so you are propably under recovered when you come to your second workout. I cant think kany other reason you would get so many less reps.
I do it around once a week as im on the anti flamingo prgram. I reckn it might be due to my weight as the past couole of weeks its kinda been fluctuating downwards and iv heard that bodyweight has a high impact on pressing strenght compared to other stuff. Not sure how true that is tho