I only have this problem when I ride my horse Apache I have never had this problem when riding other horses. I have checked my stirrups repeatedly. I try to really concerntrate on keeping my heels, though I can only have them down a bit before they start to really hurt.
The only thing I can think of is that I have knew boots that have leather soles and Apache is lazy to ride most times. It happens mostly when I'm trying to get him to canter, he has ignored me when I squeeze my calves and I give him a bit of a dig with my heel to say hey I asked you to speed up. This normally results in my feet slipping further into the stirrups and making it difficult to keep a good seat. The only way I can ever fix this is by completely stopping my horse and fixing my stirrup then starting again and this frustrates me to no end.
I have never had this problem before I have occasionally lost a stirrup but I am able to fix that whilst still riding.
I really don't know how to stop this, it only happens when I ride Apache and it has only just started happening. It makes me really frustrated during my rides when this happens I don't take it out on my horse because I know its not his fault but it is really starting to ruin my rides. Any ideas/tips would be great thanks.