[susan53]

No - as I said : it means "but I wasn't able to ".
in this case it comes out as the same which is why both are possible. In your sentence it means I wasn't able to remember - but that's because of the context, because of what was said before - I tried to remember..... Consider:
I tried to find John's skype address but I drew a blank.
in that context it would mean : I wasn't able to find John's skype address. .
The expression itself has nothing to do with memory

[fface]

Quote susan53

No - as I said : it means "but I wasn't able to ".
in this case it comes out as the same which is why both are possible.

Hi susan,

Thank you for your helpful answer. I know the two expressions don't mean the same. Do you mean the two sentences can mean the same thing because of the context?

[susan53]

Not that the phrases "mean the same thing" but that the co-text means that the full sentences end up meaning the same thing. As I said, change the verb in the first part and you change the meaning of the full sentence. But the meaning of the individual phrases remains the same : 1. I wasn't successful 2. I couldn't remember.

Another example :

The police had been investigating the murder for 6 months, but so far had drawn a blank.

Again, nothing to do with memory.