I think I know what you mean. It sounds like you plain and simply need more time in the saddle to build up your leg muscles. Earlier this year, I had no leg strength. Couldn't squeeze worth a darn, couldn't get a horse to drive off my legs. My kicks (when needed) were ineffective because my legs didn't work right. Now, after just spending that time in the saddle that changed. Some leg weights might help, but not as much as saddle time will. If you are wanting off the horse exercises, I actually recommend pilates as a lot of weakness is actually in core strength, which this will help. Also, I like to use an exercise ball between my calves as kinda the horses barrel, get into the proper position, and go back and forth between the different seats to work the muscles.
And for the kicking, I will do the "cowboy kicks" on occasion. I almost never ride with a crop - don't like them, but I will use one if I have to. For the kick, I just think about taking my lower legs off the horse and bringing them down hard on them. Sometimes it's almost like slapping the horses side to almost startle them. If you only have to do one side (canter transition, getting your horse off the leg), then I will completely take my entire leg off the horse (other leg stays on for balance), and bring that leg back on hard. Then I go back to the regular cue. It's like the reinforcement the crop gives, but without needing to carry something extra. Just my two cents worth, and hope it makes sense.