There are several issues that usually plaque a Drupal installation and here is a list to some of them below. Please note that this article mainly refers to Drupal 7 and it may or may not relate to other Drupal releases.

1. Search Not Functioning

If you find the search returns nothing then see this post for some pointers.

2. XML Sitemap Setup for Drupal Multisite

While multisite configuration has numerous benefits to make it a very attractive option, it has some important points to address for your sites to work correctly. One area is how you organize your files and folders.

This is done under "File System" setup. It is best to keep your files for different domains and sub-domains in separate folders. This will most likely save you from unnecessary headaches if you were to re-organize your sites later. If you were to lump all files for several domains into one big folder, it will be very tedious for you to pick the ones that are relevant to any site of yours. Say, when you're moving a domain to another hosting server for example.

Even if you're not moving any domain around, getting your file location organized is crucial in getting XML Sitemap to work properly.

Get started by enabling XML Sitemap for content type(s) of your choice. Say, do this at "Content types" >  Blog > Edit > XML Sitemap > Inclusion > Included.

If a node is excluded, it won't appear in the sitemap cache file. This setting will be kept as the default setting. However, you can also change the setting at post level.

In this Drupal 7 case, let's assume your domain was "example.com" for simplicity sake. So you assinged it a unique folder named "example" for all files to be kept in.

First, install XML Sitemap module from here.

Then go to XML Sitemap Setting > Advanced Settings.

This is to assign a unique folder for a sitemap per domain. You do not want this folder shared with other domains of yours as everytime a sitemap is built, it will overwrite one created before it which is wrong!

In most likelihood the sitemap cache directory should have been defined. If not enter the one that reflects your system. You need to copy this location into our next screen in Google Webmaster Tools:

This step may not be too necessary but can serve as a way to verify that your sitemap setup correctness. If all is well and good you should see something like a "YES" tick as below after you hit "Submit Sitemap" and waited for a little while (minutes):

Verify your other sites too especially if they are managed under a multisite environment using the steps as shown above.

Further reading:

Getting Drupal XML Sitemap To Work