To do it, even if it's not perfect (but working), i created a variable for the ground.
For example, with the value of "1" when the ground is with a value of 45 degres.
Then, i had an action, testing when the main sprite (the collision box) is in collision with the ground, and the value of the variable : if the value is "1", the sprite is set to a 45 angle, otherwise to 0.
It enables jump in diagonal, even if the jump is very little, but it's better than nothing, and not give the impression to the payer that the character is stuck, even if it's not very useful, as a platform game feature.
As i'm just starting my game, i will keep in mind it's a pb in my constructiuon of levels.
Sorry for my english, and i hope it can maybe help.
If someone has an idea to make "real" jumps in diagonal, i wiil be happy :)
(to resolve the pb of the hight of jump, i add in the action the thing to set the strengh of jump : for example set to 1000 when the value is "1", and to 800 otherwise. It seems to work)