Davood Dehnavi on "Sometimes CSS not loading"

Hi,
I've the following problem. My WP site sometimes don't load correclty the CSS, infact in Apache log, when the problem occurs, two lines don't appears:

[14/Jun/2016:15:18:27 +0200] "GET /wp-includes/css/dashicons.min.css,qver=4.4.pagespeed.ce.X2cvQ4DGDW.css HTTP/1.1" 304 -
[14/Jun/2016:15:18:27 +0200] "GET /wp-includes/css/admin-bar.min.css,qver=4.4.pagespeed.ce.IzXC0ivIXx.css HTTP/1.1" 304 -
[14/Jun/2016:15:18:27 +0200] "GET /wp-content/themes/mineral/css/A.prettyPhoto.css,qver=1.5.4.pagespeed.cf.hwHBzDR-Sf.css HTTP/1.1" 304 -

Normally, when it works correctly, the log shows:

[14/Jun/2016:15:21:06 +0200] "GET /wp-includes/css/dashicons.min.css,qver=4.4.pagespeed.ce.X2cvQ4DGDW.css HTTP/1.1" 304 -
[14/Jun/2016:15:21:06 +0200] "GET /wp-includes/css/admin-bar.min.css,qver=4.4.pagespeed.ce.IzXC0ivIXx.css HTTP/1.1" 304 -
[14/Jun/2016:15:21:06 +0200] "GET /wp-content/themes/mineral/css/A.prettyPhoto.css,qver=1.5.4.pagespeed.cf.hwHBzDR-Sf.css HTTP/1.1" 304 -
[14/Jun/2016:15:21:06 +0200] "GET /wp-content/themes/mineral_child/style.css?ver=1.5.4 HTTP/1.1" 200 215
[14/Jun/2016:15:21:06 +0200] "GET /wp-content/themes/mineral/style.css HTTP/1.1" 200 21432

How can I understand where is the problem?

Thanks.