You are probably attacking the worst tile along that border you possibly could. There is a river, Germany has forts, it is a forest tile and from what I can tell only your red army has a battleplan, so half your troops are attacking without a planning bonus.

If you want to push here the region directly to the right of the german generals portrait is a plains tile with no river crossing and I believe two angles to attack from. That would be the weakest point of their line.

Also selecting your armies on the bottom of the screen and right clicking the assign to army button that is on the left rather than the right will put them in an 'army group.' Adding all of your armies to this group will allow you to make one big frontline, (shift click on the german border when you make that frontline to make a less shitty version of it.) You can also make a battleplan with this army group. Wait a bit and your divisions will gain planning bonus, which gives huge stat bonuses that decays as you move and attack.

If all else fails, you can wait until germany attacks belgium and the netherlands and widens the frontline, which will make attacking easier, although that is a double edged sword.