Grief or depression could make it difficult. Compassion begins at home.
Maybe it sounded worst than it is. It was a small problem but i thought it was a good opportunity to start my compassion meditation again.
I had the same problem with my tonglen practice, in the beginning it was strong and vibrant and then it slowly became mechanical. My teacher advised me to just keep practicing. Even mechanical or flat practice is better than no practice at all. The other thing worth doing is checking your motivation. Motivation plays a huge role in the generation of compassion.
One of the reasons why i decided to start meditating on compassion again was to improve my motivation. I believe firmly in mahayana and i try very hard with prayers and so on to set up a good motivation, but if im honest to myself my motivation still is to make my life a little better, Im hoping the seeds im planting now will have good results in the future in terms of bodichitta. You are probably right, i should just keep practicing. For some reason since I've started meditation i haven't been able to establish a regularity in terms of daily and more than 15 minutes with this kind of practices ( the four immeasurable ).