Dunlop Volleys - Am I losing my grip?

I suspect that my Dunlop Volleys are losing their grip
. Although I mainly walk in boots, I do some bush regeneration field trips occasionally, for which I've found DVs are invaluable. Most of the walking is off track and usually involves rock hopping and wading through creeks for long periods, with slippery consequences. I was on such a trip last Saturday and went A over T at one point on a slippery rock (no injuries to speak of, but could easily have had a worse outcome).
The volleys are probably 2-3 years old and still look in good condition (soles are not yet particularly worn), but they seemed to me to be less effective than previously, or maybe the rocks in the creek were more slippery than usual. What are other people's experience with this?
I note that I can get replacements on special at present for under $22.
Also any opinions about different models? Mine are just the "standard" ones (in basic black) but I've seen the "International" (or maybe "Classic") type for about $44, and various "fashion" and "skate" types. Are any of these better or worse than the garden variety for grip/wear?

The volleys are probably 2-3 years old and still look in good condition (soles are not yet particularly worn), but they seemed to me to be less effective than previously, or maybe the rocks in the creek were more slippery than usual. What are other people's experience with this?
I note that I can get replacements on special at present for under $22.
Also any opinions about different models? Mine are just the "standard" ones (in basic black) but I've seen the "International" (or maybe "Classic") type for about $44, and various "fashion" and "skate" types. Are any of these better or worse than the garden variety for grip/wear?