why server not answer to some client but other client not with others

i have a webApp that some people can input data in it . About 100 people that all are company's employee can do this with no problem but one of them is facing with a problem with my webApp . when he/she loads a page of application or query data from server , server does not answer him/her . By replacing his/her computer with another computer , problem will be solved . it depend on the computer . but why this happen . this problem takes place in some special computer and has no related to program .is there ant setting in window or pc that causes this problem? . i wrote this program with laravel 5.2 . i have no idea about it and am so confused . any idea is appreciated .

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