That's normal for single channel (one RAM card), it will be better with two RAM cards in dual channel.
It's better to have matched RAM cards, with unmatched cards it's possible to be unstable. Download CPUZ
and you can get the supplier and part number from the SPD tab.
Try to get this exact brand/model with the same voltage and JEDEC info.
There are some RAM manufacturers that will guarantee compatibility with your laptop, they can't guarantee compatibility with your current RAM card. Try searching the Crucial site
, they and a few others have a good return policy if there are any problems.
It's always recommended to get matched RAM kits, which will be more stable.