It is true that you can lose the lowest notes throughout the day, but 10 semitones is quite dramatic! It makes me wonder if you are straining your voice during the day. If you are, it would help to address that.
Aside from that, the best thing I can suggest is spending some time practicing low notes specifically. Lower notes are generally easier for people to hit than higher notes, but the lowest notes can actually be quite tricky. At a point it's like they require their own technique. Some things to pay specific attention to:
Breath support -- obviously, all singing requires good breath support, but I would expect you to need much more breath support for the E than for the A.
Posture -- In my opinion, it can be far from perfect for most singing as long as you have the breath support... but for me it makes a difference on the lowest notes.
Larynx position -- the larynx naturally raises a little as you go up and lowers a little as you go down, but you want to make sure it's relaxed. Make sure not to push. Pushing the larynx down as you sing lower is a way of getting a bigger sound (probably unconsciously), but it limits your range.
Volume -- the lowest notes can get pretty quiet... which you might not notice first thing in the morning, but then after a day of hearing louder sounds and singing higher passages at much louder volume, it's natural to come back to those low notes and try to push them out. (Well, it is for me at least. It's not something I'd do on purpose, but reminding myself to shift gears when going lower has helped.)
Additionally, when practicing low notes, focus on getting a clear tone first, even if it's barely audible. When you get better at producing a tone consistently, then it is much easier to make it a little bit louder. (It'll still be quiet, but it'll be better than "barely audible.")
When you're doing exercises, make sure to visit your lowest notes before your highest ones... and if you go back down to low notes throughout the warmup, it helps retain them (even though you still lose a few semitones... 10 is much more than I would expect). The rest is just practice and attention to that part of your range.