Not allowed to make all champions league substitutions

Having tried again in the 2nd leg, it let me make all 5 substitutions when I had only made 2. I have only come into this issue when I have made 4 changes and want to make the 5th and final on its own.

How many stoppages did you use to make the four substitutions?

While you're allowed five subs (in a regular match), you're only allowed to use three stoppages. So you can't make five subs one player at a time, for instance. But you can make 3+1+1 or 2+2+1 or however you'd like.

As mentioned above you can only make the subs in three intervals. This is by design.