Did you keep the temperature low? Organic compounds are prone to carbonization when in contact with concentrated H2SO4, and the higher the temperature the faster the carbonization process happen.

http://pelagiaresearchlibrary.com/der-chemica-sinica/vol5-iss5/DCS-2014-5-5-67-70.pdf

Don't use sulfuric acid. The yields are terrible and you get a black sticky tar.

http://www.hindawi.com/journals/oci/2012/306162/

This procedure with BaCl2 looks good. It's a fairly facile reaction with Ethyl acetoacetate.

You could also try AlCl3 or MeSO3H if you haven't got any BaCl2

Thank you 

There are ways to avoid sulfuric acid. See the attached publication

This is procedure for preparing 4-methyl-7-hydroxy coumarin. First in separate beaker (100 ml) must to mix Acetoacetic ester and resorcinol. After that, in separately beaker must to put conc. H2SO4. Both cups must be cooled in mixture ice-salt below 00C. After that, must add very slowly (in period of 30 min). acid to organic compounds and stir with glass road. After full adding, reaction mixture must be kept cooled for period of 2 hours. After that, must to add water and ice (50 ml) and stir with glass road, until the ice melted. After additional stirring, the reaction mass must to become crush able, and than must to filter in Buchner funnel. After that the resulting mass must be re-crystallized from 50% ethanol.

This compound was laboratory exercise for our students from organic chemistry.

 Also H2SO4 and POCL3 at 20 °C

J Med Chem 1991 34 2410-4 and ref therein