Nasus Klepto Bug?

Hey guys, Mr. Shield here. As a Nasus main I've really had a lot of fun using klepto for a lot of different matchups. Recently, however, I've noticed some odd things about it. The description of klepto says nothing about an internal cooldown for the rune, so logic would conclude that the "cooldown" of klepto is based on the cooldown of your abilities and your ability to weave in auto attacks between your spells during your rotation. This does not seem to be the case with Nasus as several times during one spell rotation while weaving autos between each ability I have noticed that klepto will fail to proc 1-2 times during each rotation. Thanks for reading, I look forward to hearing your thoughts.