Library Form - Book Issue Return - Synchronisation -Stock upadtion -issue -Return

Library Form - Book Issue Return - Synchronisation -Stock upadtion -issue -Return

I have 3 forms for library management - 1) book stock entry 2) Book Issue 3) Book Return

book stock has fields like bookId , bookname ,author etc  ,
In bookissue form I am fetching,  bookid , memberid  as lookup values . when a book is issued to a member the respective book is subtracted from stock and the report shows the  details of the member and the books taken by him or her .
in book return  - when a member returns the books it has to be updated in the stock . (each member is allowed only 3 books at a time ) .


memid = LibraryMemberandID[ID == input.MemberID].MemberName;
bkid = LibraryBookStockEntry[ID == input.BookIDToIssue].BookName;
Avlbk = LibraryBookStockEntry[ID == input.BookIDToIssue].Numberofcopies;
now the logic is based on member Id the bookId is fetched from book issue form - ( not able to use fetch because the bookid is already fetched from stockentry into book issue form and when i try to fetch it again in to book return form it is not available .)

so I used drop down and fetched through collection variable bookid of user
mybooks  =  LibraryManagement_BookIssue  [MemberID == input.MemberID];

the issue is ( book return  to update stock)
bkrtn  =  LibraryBookStockEntry  [BookName == input.BookToReturn];
bkrtn.Numberofcopies = (bkrtn.Numberofcopies  +  1);  ( is not working)

but
avlaftissue  =  LibraryBookStockEntry  [ID == input.BookIDToIssue];
avlaftissue.Numberofcopies = (avlaftissue.Numberofcopies  -  1); ( is working)

can any one help on this ?