Textboxes Not Keeping Value?

I have a big clunky spreadsheet that I thought I'd tidy up by adding a userform to keep the info

There's a combination of textboxes, option buttons, listboxes and a multipage. Everything seems to load correctly, and selecting an item in a listbox correctly fills the textboxes, but when I change the value of the textboxes, only 2 out of 4 holds that value and sends it to the correct cell.

The code that is supposed to send all the values is;

VB:
Private Sub CommandButton1_Click()
'check that a row has been selected
If Range("K3") = "" Then

[Code].....

The culprits are TextBox3 and TextBox4.

I've tried changing the names, I've checked that there are no other textboxes pasted on top, I've even tried adding in a MsgBox line to show the value of the textbox; if the form loads and the box grabs a value of "ABC" from the spreadsheet, and I change it to "DEF", when I hit the button to execute the above code the textbox goes straight back to "ABC".

But it's not an issue with TextBox2 ...

Other parts of the code include a UserForm_Activate block and 5 x ListBox_Click's.