'Missing artifact' and 'Failed to read artifact descriptor' errors on Spring MVC project

I'm getting multiple "missing artifact" and "failed to read artifact descriptor" errors on a Spring MVC project, for example:

Missing artifact com.google.guava:guava:jar:20.0
Failed to read artifact descriptor for com.brooksandrus:swfheader:jar:1.0

I'd like to know:

What exactly do they mean
How to solve them

What I think of these errors:

Also that the fact that I am running the project behind the company's firewall could have something to do with the maven clean install failing with the following error: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
Actually, this was one was solved having each required certificate installed in the cacerts file.

What I have already tried:

Update Maven Project having Force Update of Snapshots checked.
Deleting the entire .m2 folder and then trying a maven update.
Followed this SO solution to add the certificates on cacerts file and then rerun the first two steps. This is actually how I solved this error unable to find valid certification path to requested target although it didn't help with the main problem that I mention about the 'Missing artifact' and 'Failed to read artifact descriptor'.

None of these approaches had any effect on the problem.

Another important fact is that besides all these errors, Tomcat launches with success and the project is working properly.

Here is an example part of the pom file.

<?xml version="1.0" encoding="UTF-8"?>
<project
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
    xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <modelVersion>4.0.0</modelVersion>
    <groupId>gr.xxxxxx</groupId>
    <artifactId>xxxxxx</artifactId>
    <version>2.2.7</version>
    <packaging>war</packaging>
    <name>xxxxxx</name>
    <description>xxxxxx</description>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <spring.version>4.3.23.RELEASE</spring.version>
        <spring.security.version>4.2.12.RELEASE</spring.security.version>
        <hibernate.version>4.1.4.Final</hibernate.version>
        <build.plugins.plugin.version.compiler>3.3</build.plugins.plugin.version.compiler>
        <build.plugins.plugin.version.javadoc>2.10.3</build.plugins.plugin.version.javadoc>
        <junit.version>4.11</junit.version>
        <hamcrest.version>1.3</hamcrest.version>
        <tika.version>1.7</tika.version>
        <mockito.version>1.10.19</mockito.version>
        <doclava.version>1.0.6</doclava.version>
        <adwords.version>4.7.0</adwords.version>
        <hikari.version>3.3.1</hikari.version>
        <mariadb.version>2.4.2</mariadb.version>        
        <thymeleaf.version>3.0.11.RELEASE</thymeleaf.version>
        <stagemonitor.version>0.88.10</stagemonitor.version>
        <javamelody.version>1.79.0</javamelody.version>
        <dynamicreports.version>5.0.0</dynamicreports.version>
        <timestamp>${maven.build.timestamp}</timestamp>
        <maven.build.timestamp.format>dd/MM/yyyy HH:mm</maven.build.timestamp.format>
    </properties>
    <repositories>
        <repository>
            <id>jsqlparser-snapshots</id>
            <snapshots>
                <enabled>true</enabled>
            </snapshots>
            <url>https://oss.sonatype.org/content/groups/public/</url>
        </repository>
        <repository>
            <id>mulesoft-releases</id>
            <name>MuleSoft Releases Repository</name>
            <url>http://repository.mulesoft.org/releases/</url>
            <layout>default</layout>
        </repository>

Any ideas on how to deal with this are appreciated.

Solution

First of all, it's always a good idea to run mvn from command line. It isolates the problem from your IDE and shows logs when maven resolves artifacts.

Also, you can add -X CLI option which produces more debug output and you should see some useful errors:

mvn clean install -X