SQL query timeout error

I am using vb6 application with Sql Server 2012. Very often I get timeout error.

I have 700+ tables in the database. Some tables i am using in query have 20K + rows (some have very few records also).

I use begin transaction and commit transaction.

I have several small small queries in the transaction. 

Just for example,

Suppose i am saving invoice created for customer, after i click SAVE button,

 first I go to 1 or 2 other forms to calculate totals and get accounts etc  etc. so load those forms and get the data.

Then checking dates and some other data in other tables.

Then i delete existing invoice first, then insert new invoice in say 4/5 tables.

I get timeout error [odbc sql server driver]timeout expired. I have give commandtimeout = 600 and connectiontimeout=600 already. I am using ADODB.Connection. My tables are not indexed.

How can i eliminate or atleast minimise this error.

h2007