Very late to respond, I hadn't seen this until now. Juan, I found the answer soon afterwards. I believe I was saving to the .axf instead of the .apm. Newbie mistake I've been using ArcPad for years now, and really like it. It is so much more customizable than Collector. Wish you guys could either move ArcPad onto iOS (which IT is pushing us to do), or make a new product supported like Collector that has the customizable features to it. As it is now, we are moving to Collector because of IT issues and it just looks like ArcPad is going away (boooo!), but the transition is taking over a year to try to make the program work for us.
I feel your pain wrt ArcPad being phased out. I don't know if you're aware of this product, so I thought I'd let you know that I have my eye on AppStudio for ArcGIS. I hope it may become an option for smarter-than-Collector data collection in the future. Right now it seems mostly focused on data display, at least in terms of the out-of-the-box templates available.
Yes, I am aware of it, and was really excited to test it out when I saw it in action at the '16 UC. It requires a level of coding expertise to make it work for us that I haven't been able to find the time to develop or at least test out, though. ArcPad certainly requires coding to do what we do with it, but that's just customizing, not building an entire app! But, thanks for the nudge. I think it's time to start playing with it. Thing is, ArcPad does it all - such a shame to have to go back to the drawing board!