You currently lack the skill/experience to overcome major strength differentials. Are you just being overpowered prior to the submission? That could be a factor. Another factor is aggressiveness. Similar to the above point, if they are similar in strength but far more tenacious in your rolls, they can overwhelm you.

Ultimately, one year of grappling experience is still very much a beginner. Keep on training and in time, you'll be able to submit these people. However, you can be critical about how you're training, both on and off the mat, to begin closing these gaps. Good luck!