Hii

This might be because of the heavy traffic on the site nowadays. Almost all the candidates are trying to finish up their registration and so the site has a very heavy load these days.
You are not the only 1 facing this issue, there were many others with same problm. You can try to refresh the page i.e logout once and login again. Then check if the process has been updated. Also wait for 24 hours until the payment slip gets generated. 
Even after 24 hours if there is no update, i would simply suggest you to make the payment again and complete your process. Duplicate payments would be refunded to you within 3-5 working days after the  completion of the process.
Also you can contact AIIMS authorities on the following helplines - 
Phone - 011 26588500 , 26588700, 26589900
Email - exams.ac@gmail.com , aiims.mbbsexams2019@gmail.com

Hope this helps
Thanks!