Thanks for contacting us, obviously there is some kind of an issue with your transmission, when it comes to transmissions it is very complicated to pin point the problem, especially when you can’t get any fault code on normal diagnosis procedure..as you describe, if you are a do it yourself type of guy or if you have an independent service provider that willing to work with you, I would recommend flushing the fluid, replacing filter and gasket and refilling fluid at correct level, your problem can be caused for few various reasons, I am not going to get into the dynamics of a transmission nor I am going to explain how it works, you can find many YouTube’s explaining that however, I will tell you that low fluid level can cause the transmission not to engage properly, since it is a closed transmission, you will find it a very challenging job to do it yourself…if replacing fluid did not correct the issue and under the assumption that you get no fault codes, I would recommend having it inspected by a repair shop.. it may be your torque converter, valve body(sticky valve) updates on your software or a bad solenoid, whichever it is, you will need a professional opinion. thanks.