Sticky bottom syndrome in rabbits?

I have two female rabbits, they both have sticky bottom syndrome, im just wondering what im doing wrong and what i need to change. They have the run of our garden which they are in from morning until night, there are no plants but there is gr, i try to let them have as natural a diet as possible, i put hay out for them more so in winter for them to eat as they are not really interested in the summer but they have plenty in their hutch that i know they eat, i do give them some veg but am trying to not have them rely on that so much so they will eat more hay. They are large lop eared rabbits and one in particular has a large dewlap, but she is the more active of the two, ive never been told by the vet they are overweight they are just large. They have a small amount of pellets at night which i purchase from my vet and have recently started adding small amounts of a lighter version just to try and keep their weight under control incase that is an additional factor in this, im just wondering what it is thats causing the droppings to collect like this, what im doing wrong and what i need to alter, any help would be much appreciated, thanks!