Yes, if a candidate has missed filling up the Application Form or its form rejected for the January session, he/she can fill up the Application Form for the April session at the time of opening of the portal for that session but the thing it totally depends on the reason for which your form gets rejected.

If your form is rejected because of not fulfilling the eligibility criteria then you must ensure that in april session you should fulfill it otherwise no use of filing form again its again gets rejected.

However,you can get all details of form filing here

https://engineering.careers360.com/articles/jee-main-application-form