Varnish not working

The other day while working on our site unexpectedly going down, I was informed that Varnish was not working on I’ve disabled all plugins that we don’t use. That has not solved this issue. How do I go about determining what is preventing Varnish from working?

Below is the output from I am curious about two things that I’ve highlighted - why the expiration date is in 1981 and what the PHPSESSID is.

The url we checked:
HTTP/1.1 200 OK
Server: Apache
[color=#1E90FF]Expires: Thu, 19 Nov 1981 08:52:00 GMT[/color]
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
[color=#1E90FF]Set-Cookie: PHPSESSID=xLJ4yzXLUllBjTCdjSTb22; path=/[/color]
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Type: text/html; charset=UTF-8
X-Cacheable: NO:Not Cacheable
Content-Length: 6622
Accept-Ranges: bytes
Date: Wed, 14 Jan 2015 17:16:27 GMT
X-Varnish: 466909778
Age: 0
Via: 1.1 varnish
Connection: keep-alive
Link:; rel=shortlink

What plugins do you still have enabled? It’s possible that you’re using a plugin which is not compatible with Varnish caching.

Hey Andrew,

Here’s a list of plugins. Do any of these look like trouble? I’m new to WP, thanks for your help.

Installed Plugins: Capability Manager Enhanced by Jordi Canals
Kevin Behrens version 1.5.2
DreamObjects Backups by Mika Epstein version 3.5.1
Google XML Sitemaps by Arne Brachhold version 4.0.8
IgniteWoo Updater by IgniteWoo version 1.1
Jetpack by by Automattic version 3.3
MailChimp for WordPress Lite by Danny van Kooten version 2.2.2
Ninja Forms by The WP Ninjas version 2.8.13
Regenerate Thumbnails by Viper007Bond version 2.2.4
Simple Custom Post Order by Sameer Humagain version 2.2
SP FAQ by SP Technolab version 2.2
Varnish HTTP Purge by Mika Epstein version 3.5.1
Woo Product Importer by Daniel Grundel
Web Presence Partners version 1
WooCommerce Auctions by version 2.4.1
WooCommerce Print Invoice & Delivery Note by Triggvy Gunderson version 4.0.2
WooCommerce - Watermark by David Baker version 1.0.4
WooCommerce Sequential Order Numbers by SkyVerge version 1.3.4
WooCommerce Stripe Connect Payment Gateway by IgniteWoo version 2.2.2
WooCommerce by WooThemes version 2.2.10
WooThemes Helper by WooThemes version 1.4.1
WP Google Analytics by Aaron D. Campbell version 1.4.1
WP-Mail-SMTP by Callum Macdonald version 0.9.5

Figured it out - it was Ninja Forms. Replaced it with Contact Form 7 and things are looking better.