I find that I'm always unhappy with the results if I don't take the time to make it right. If it's just ok when I start, then when I'm done it's not right and I'm sorry I hurried.
That of course doesn't help you fix the problem.
Prism gave you a great answer. I usually find once I clean the machine, change the needle, rethread the machine and have a cup of tea it's working great.
Good luck
Cindy