Maybe your CV its not matching their requirements. Also i would recommend you to attach a cover letter each time you apply to a job. Try to avoid to use a standard cover letter for all the jobs, personalize each time according to the position you apply. Best regards.
If your CV is to long or to short hiring managers see this as either to much information and they don't have time to read through all your great skills and if it's to short it either shows lack of skills or lack of effort. Without know your CV I can not truly say what is the issue with your CV. Good Luck with all you do.
