Disuniting whilst going towards a jump

[msjumper83]

Hi im wondering if anyone has some advice for me i have a 9yr old dutch warmblood cross tb. Who i have for showjumping, he has jumped bsja ect sometimes when i go up to a jump he gets a bit keen i try and check him back and he disunites not all the time just occasionally he doesnt do it on the flat maybe i am doing something wrong like checking him back to much ?. Is there any exercises i can do with him to help it or am i doing something wrong ? :

 

[Weezy]

Stab in the dark, but do you jump him in a gag?

 

[msjumper83]

hiya yes i do