Post Upgrade some instances are giving http 503 error in PI

Hi,

We have recently upgraded our SAP PI system from 7.4 sp13 to 7.5 sp13 which has 4 application servers. But post-upgrade we can see 2 application servesr are giving error HTTP 503 on start page and others 2 are working fine.

Did anybody face such type of issue? Please help in this issue.