I'm missing the problem. You say you haven't slowed down and you're not complaining of being tired. Your only complaint is your HR doesn't seem high enough. It's normal for HR to get lower for a given power output with increasing fitness. I suspect it was easy to get your HR up on the trainer because you can never have a big enough fan to provide the kind of cooling you get when riding outdoors. That doesn't mean you were any faster or putting out more power though.
Ideally, you would have a powermeter and be able to measure your progress. Alternative measures would be to use your trainer or time yourself up a hill. If the trainer has a known speed-power curve you can compare your speed now vs what you were doing in the winter.
My other thought is regarding your composition of intensity during the week. What does a typical week look like for you? You should have 2 or 3 hard days and the rest should be easier. If you're trying to ride the same pace all the time it doesn't surprise me that you aren't able to get your HR up. You'll still improve but not as fast as you might if you mixed it up more.