Multiple items in same year by same author not listed correctly

I’m having the same problem

Please, what I need to do to correct this problem?

Wrong letters when I cited references from same author and same year!! 

I’m having problems: It’s appearing wrong letters when I cited references from same author and same year!!

Corrected expected:

(Gottsberger and Silberbauer-Gottsberger 2006a; b),

 

But is appearing in this way: with letter K, instead of “b”: 

(Gottsberger and Silberbauer-Gottsberger 2006a; k).

 

Expected: (Paulino-Neto 2014a, b, c)

Appearing:  (Paulino-Neto 2014a, h, n)

references

Gottsberger 1989a. Comments on Flower Evolution and Beetle Pollination in the Genera Annona and Rollinia (Annonaceae). Plant Systematics and Evolution 167: 189-194.

 ------. 1989d. Beetle Pollination and Flowering Rhythm of Annona Spp (Annonaceae) in Brazil. Plant Systematics and Evolution 167: 165-187.

Thanks for your attention

Best wishes