Not sure sure if it's a bug. But I can't unlock specific plants,

There are 5 types of plants and not all plants are unlocked by seeds.  

1). The original world plants, which were originally unlocked by completion of a world level.  There was a change made in last year's update grouped them into seedium.  (see item 4 below)

2). Cash premium plants.  These plants unlocked only with real cah, you can find them in the store, under the (green) Plants tab.  

3). Gem premium plants. (Gemium)  These plants unlocked only with gems, similar to above, you can find these plants in the store.

4).  Seedium plants.  This applies to Arena plants, new and old.  Once you have the required amount of seeds, the plant is unlocked.  Last year's change has also grouped world plants to this class so player can unlock plants from the future worlds without requiring to complete the level.

5).  Mint plants.  With the exception of Enlighten-mint which was assigned to seedium, all Power Mints are unlocked with the actual power mint plant itself, it costs 100 mints (tokens).  They are normally on a rota from the store.  

Now, what plant or plants are you trying to unlock?