This article comes to us from the ever generous and butt kicking Peter, of 'Straight Outta Caledonia'. Make sure to give his blog a gander! It's a superb blog about the finer things in life...just kidding. It's about Caledonia and Peter's fun adventures of being a WarCor.
Lately I have been playing more casually which means I have made my army lists more for fun than just for winning. As an unexpected side effect it helped me to analyze better why I have lost games and I thought it would make for a good article. I can't guarantee this will cover all possible reasons why games are lost but it should be the most common reasons.
That said, it is possible you are rolling your dice wrong, as in, you are not actually rolling them. Never just drop your dice, make sure they roll on the table or in a box or any kind of surface where you roll dice. If you drop your dice you either get a number you don't want, and if you do get the numbers you want it can make your opponent feel like you are cheating with your rolls.
Expecting to win bad odds
I am not saying you need to think about math during games (my philosophy is that everything is a 50/50 chance, either it works or it doesn't) or don't take risks, but if you take risks with bad odds often it can be a reason why you are loosing face to face rolls and in extension, the game. Hail Mary rolls should be your last resort.
Not playing the mission
Everyone has a different way of how they spend their Order pool but I would simply recommend that in each of your three turns you spend some Orders towards doing the mission. Of course you will always have the enemy units to deal with, but while you can afford to ignore your enemy, you can't ignore the mission and expect to win. Keep in mind that in certain missions you can go the whole game without killing a single enemy unit and lose your whole army but still win on objective points.
Not prepared for the mission
If you don't know the mission it can be very hard to beat your opponent, especially if they do know the mission. I understand that not everyone can practice often but it can be helpful just to sit down, read the mission, think about your army and how you achieve the objectives. Your plan will not always work but I do very much believe that you will play better if you know what to expect from the mission.
Army not flexible
I know from experience how frustrating it is not being able to deal with an enemy unit, and equally I noticed how frustrating it was for my opponents when my two Cateran Snipers were pinning their whole army. I would recommend to always expect that your opponent will have snipers, high burst weapons (HMG, Spitfire, etc), Total Reaction Remotes, Camouflage units (or ODD) and Heavy Infantry (or other units with high ARM and more than 1 Wound), and have a mix of units in your own army that can deal with those threats. That way you should have a better chance of overcoming obstacles during your games.
As I mentioned earlier, this list isn't exhaustive but I hope that it at least gave you an idea of what might be going wrong in your games. We all like to win games but when you lose, it is always helpful to talk with your opponent about the game and what they thought about your army. Chances are they might see a flaw that you don't. Be critical of yourself and think about what went wrong so you can learn from it.