I don't want to include Hrithik in my answer because he doesn't have consistent movie releases.
The main reason Khans are lagging behind Ajay & Akshay is because they don't know what genre is trending and what is being rejected. Ajay in 18 had Raid, film is based on a real life incident, in 19 he had 2 releases both were comedies. His upcoming films are Tanhaji (historical ), Bhuj (war; patriotic film) and Maidaan (biopic), he'll also be making a cameo appearance in Sooryavanshi.
Akshay on the other hand is all over the place, he had Kesari (historical), Mission Mangal (real incident, patriotic, women empowerment). His upcoming films in 19 are Housefull 4 (comedy) & Good Newwz (drama; can't put a genre on this) In 2020, Sooryavanshi (masala); Laxmmi Bomb (horror comedy); Prithviraj (historical) & Bachchan Pandey (masala).
What do the Khans have?
SRK - no idea, most likely action film
Salman - Dabang 3 (masala); Radhe (masala); Kick 2 (masala)
Aamir - LSC (comedy-drama); Mogul (biopic)
Khans don't have consistency and they are always last in the genre race, they arrive when the field is already empty (especially SRK) and they always want to make an event film. They need to have regular releases and need to keep the budget in check.
I'm an SRK fan so I'll say this about him: I think he is living in a bubble where everyone just says "you are the best" and "you can do no wrong"; his fanbase keeps defending him, they need to start being honest, his last few films haven't been upto par, he needs stop doing vfx heavy films and Aditya/Yash Chopra love stories.
PS: BO collection means very little to me, I'm not going to get a portion of the money if I defend my favorite in these forums or twitter.